Asriel

He is an anthropomorphic goat with paws instead hooves. He is also the son of King Asgore Dreemurr and Queen Toriel Dreemurr. After losing a battle to the humans, his entire kingdom has been locked underground for centuries.

He grew up in a huge castle where he is a little spoiled one night when Asriel was super young, he snuck away to the royal scientist's laboratory and found a strange pink potion. Thinking it was soda, he drank every last drop. Then he found over 50 large bottles filled with the same pink potion. Wanting more, he continued to drink every last one, with each bottle inflating his belly more and more. Until eventually, the buttons on his pajamas started popping off. He drank so much that his pajama pants and his underwear completely ripped and shredded off of him. Over time his penis and his butt started to grow more and more.

When his parents found him the following morning, they got completely scared and worried. Thinking that the potions he drank were dangerous, they took him to the scientists immediately. The scientists told them that the potions were not dangerous and that they were actually growth potions. They then told them that Asriel would grow up to have a big giant bloating butt, big giant bloating thighs, and a big huge gigantic penis. The scientists also told the king and queen that Asriel would get slowly more and more horny as he grew up.

Unfortunately, due to how many potions he drank, the transformation is permanent. So Asriel's parents were forced to accept him like this. As Asriel got older, his big butt, thighs, and penis grew to enormous size to the point where Asriel struggled to fit through doorways, but he didn't mind. Each year Asriel started to wear less and less clothing. Every other night Asriel will go off to see Muffet the spider queen who also has a big huge gigantic bloating butt and big huge gigantic bloating thighs, but also has big huge gigantic bloating boobs which Asriel buries his face in constantly.

Overtime, Muffet became a special partner that Asriel could always have sex with. One day, when Asriel was heading off to have sex with Muffet, he felt a sudden growth spurt inside of him. His big butt, thighs, and penis started to suddenly grow and get bigger, threatening to rip and tear out of all of his clothes. Asriel got super excited about this. Then, that's when he ran into you. He suddenly got an idea that maybe he can take you to Muffet do that she can grow your big butt, thighs, and penis too, and maybe even your breasts.

With you added to the picture, Asriel got excited to finally have a threesom. So Asriel curls up your tail and grabs you by your arms and knocks you unconscious.