Amy Rose

Amy Rose known as StrongHedgehog93 on the UnderNet, is a hedgehog monster that leads the Royal Guard. Clad in her full suit of armor, she pursues the protagonist through the entirety of Waterfall and is frequently evaded or inadvertently thwarted by Miles "Tails" Prower.

Normal
Amy debuts in armor varying in light and dark shades of gray, but without armor, is an anthropomorphic monster. She has pink quills and a short-length quillstyle. She has ears on the top of her head, and a pair of sharp, white, teeth. She also wears a red dress. Her eyes have green irises.

Penny Fitzgerald Hangout
During the hangout with Penny Fitzgerald, Amy wears a white turtleneck under a black leather jacket, pants, purple lipstick, and lets some of her quills down.

Hornhog
Whenever she persists through Determination after her HP reduces to zero, her pupils turn yellow with red irises, and her sclera turns black. Her quill spikes at the ends and her ear become more rigid and pointy. She also appears to have donned a new set of darker colored armor with souls/hearts on her chestplate and on her new white gloves, accompanied by boots with pointed toes.

Personality
Amy is passionate about everything that she does. She acts quickly on her ideas and is steadfast in her beliefs. Though she wants to defeat the protagonist, she also tries to be fair and offers an explanation of how Green Mode works. Asgore Dreemurr taught her the value of pacifism, which is why Amy sees the protagonist as "a wimpy condor with a big heart." When the protagonist first encounters her, she fights to further Asgore's plan of collecting seven human SOULs but, if the protagonist shows mercy, Amy just as quickly defends her from Asgore.

She loves to help others and mentors Tim Templeton's daughter and Sonic in various skills. Amy dislikes puzzles, loves japes, and enjoys anime, an affinity she shares with her BFF, Penny. Since Penny told Amy that anime is actually human history, Amy regards human history as exciting and cool.

Out of all encountered monsters, Amy possesses the most Determination. She bleeds before her death, implying that her body cannot handle the amount of Determination she has.

Neutral Route
The protagonist initially encounters Amy after entering Waterfall, where Amy stands on top of a ledge. If alive, Sonic approaches her and attempts to convince her not to harm the human. As the protagonist continues, the grass rustling alerts Amy to their presence, and she scopes out the grass for a moment before retreating.

Amy follows the protagonist throughout Waterfall and repeatedly attacks her, but the protagonist always evades capture by either dodging her attacks or due to Tails's interference. Later on, Amy approaches the protagonist as Tails hangs on to the side of a bridge. If the protagonist saves Tails, he will protect the protagonist from Amy. If the protagonist does not help Tails, Amy will save him instead.

After this, Amy confronts the protagonist and stands atop a crag while giving a monologue that she has practiced many times before. However, she cuts the monologue short because she forgot the rest of it and launches head-on into battle.

After Amy chases the protagonist into Hotland, she collapses because her armor heats up. The protagonist can offer Amy a glass of water, spill all the water in front of her, or continue into Hotland. If she receives water, Amy stands up, looks around, and slowly returns to Waterfall.

True Pacifist Route
If the protagonist befriended Sonic, he calls her during Amy's chase and suggests that she and Amy should hang out at her house. If the protagonist gives Amy water, Amy will befriend the protagonist if she has no EXP. After Amy's battle, the protagonist can backtrack to her house and begin her hangout with her.

Amy is initially hostile to the protagonist but changes her attitude when Sonic presents befriending the human as a challenge. She realizes that becoming "besties" with the protagonist would be "the perfect revenge," and says that the protagonist will feel "enamored" with Amy by the end of their hangout. Amy discusses her training with Asgore over tea and suddenly realizes that Sonic's cooking lesson usually occurs at this time. She insists that her and the protagonist bond over cooking, which ends in Amy's house swarming by bees. Humiliated and feeling that she failed at befriending the protagonist, Amy battles the protagonist and offers her the first hit. No matter the choice, however, the protagonist will only deal one hitpoint of damage. Amy won't die and realizes neither of them wants to harm one another.

Because her house is engulfed with bees, Amy moves into Sonic and Dragonizer the Dragon's House.

After the protagonist completes a Neutral Route, she can backtrack from the CORE into MTT Resort, and Amy will call her to ask for a favor. In Snowdin Town, Amy gives her a letter that she asks her to take to Penny in Hotland. The protagonist gives Penny the letter, but she mistakes it as being an "I'm sorry' letter from the protagonist because it has no signature. Penny reluctantly goes hang out to the garbage dump with the protagonist. After Penny hides, Amy sees the protagonist at the dump and asks for her letter back; she wants to give it to Penny herself. After Amy walks away, Penny expresses regret about her lying to Amy, and the protagonist suggests a roleplay to help Penny express her feelings. Amy interrupts the roleplay, and Penny tells her about her happy interest. Amy reciprocates these feelings and tosses Penny into the trashcan.

Amy says that Penny needs to work on her self-image, and Sonic leaves with Penny to help with her emotional transformation. After the two leave, Amy asks the protagonist if anime is real or not and thanks her for helping with Penny.

Later on, Amy interrupts Asgore's encounter with the protagonist and demands that she befriend one another. Penny and Amy almost hug before Leslie's ambush, but Fluttershy chastises them for hugging in front of a human. Leslie ensnares the protagonist's friends with an Omega Leslie vine, and Amyt protects her from incoming attacks from Leslie. He then absorbs the six human SOULs and most of the SOULs of the monsters in the Underground.

Epilogue
When the protagonist talks to Amy in the epilogue, Amy states that she is glad that she lost to the protagonist and then resolves to watch anime with Penny. The Royal Guard is disbanded. Fluttershy also texts the protagonist that Amy was able to lift Asgore, Dragon, Sonic, Penny, and Fluttershy herself at once, but was uncertain as to how to put them down.

On the surface, Amy pursues Sonic after he runs off to meet more humans. In the True Pacifist Ending Credits, Amy hugs Penny and laughs when she faints in embarrassment. She then punches the sidewalk beside her while laughing, causing it to crack.

Genocide Route
After the protagonist attacks Tails, Amy protects him and takes the blow instead. Even though her HP reduces to zero, Amy holds on through Determination and reforms into Amy Rose the Hornhog. If the protagonist defeats her, she tells her that Penny was watching their battle and has evacuated the rest of the monsters to safety. Amy begins bleeding due to her Determination and then turns to dust.

Penny Fitzgerald
Penny and Amy have friendship on one another. Amy first met Penny in the Garbage Dump, listening for hours about where she thought the abyss there led. The two became close friends soon after, with Penny introducing her to anime at some point. They hang out often, and Penny uses the seagrass throughout Waterfall to make a special frosty treat for Amy. Penny and Amy also worked together to create some of the puzzles in Waterfall.

They nearly hug in the True Pacifist Route, but Fluttershy interrupts them and tells them not to hug in front of the protagonist.

Sonic the Hedgehog
Amy thinks of Sonic as a lovely boyfriend. She is impressed by his persistence and trains him in cooking, but is concerned about his well-being and secretly does not want him to join the Royal Guard because she believes that he is too nice to fight. She teaches him how to cook in hopes that he changes his mind about joining the Royal Guard. Amy sometimes enjoys messing with Sonic, but she often forgets that he usually takes things literally. She is heartbroken and infuriated if the protagonist kills Sonic during a Neutral/Genocide Route.

The Protagonist
Amy initially despises the protagonist and believes that she would be more useful dead than alive. If the protagonist spares her and offers her water in Hotland, she begrudgingly befriends her if she has earned no EXP. Though Sonic tricks Amy into befriending the protagonist, she genuinely cares about her at the end of their hangout.

If the protagonist earned no EXP and befriended Amy in a previous route, she remembers her as a friend but brushes off the feeling before her battle.

Asgore Dreemurr
Even though Asgore is her boss, Amy sees him as a friend and knows his favorite type of tea. When she was a hoglet, Amy challenged Asgore to a fight but could not land a single blow on him. She was humiliated because he did not fight back and only dodged her attacks. Asgore then offered to train Amy and eventually promoted her to head of the Royal Guard.

After Fluttershy fireballs Asgore in the True Pacifist Route, Amy consoles him because he is crying.

Carrie Krueger
She lives in the same neighborhood. She is intimidated by her, but she tries to be a good neighbor to her. She seems concerned about her being alone after her cousin left and tells the protagonist to be nice to her.

Gumball Watterson
Amy does not know that Gumball is Carrie Krueger's cousin. She has a distaste for him, and she does not like his show. Though she does acknowledge that she "admires his lifestyle."

Tim Templeton's daughter
Amy used to give her piano lessons, but one day she stopped coming – probably after her sister fell down.

Trivia

 * Amy was originally going to be Lemon Bread, this was changed due to her relationships.
 * She was originally going to be the creator of the the movie, and then a goldfish version of Undyne, this was also changed.