When I first commenced writing Exit the Fly, I knew that the interaction between Baxter and the Turtles (and by extension, Splinter) would be one of the most important elements of the story. It would be largely through their kindness and encouragement that Baxter would begin to realize his worth after his horrific cross-fusion. And of course, it would ultimately be by coming to try to save them from Shredder and Barney, and trying to save Barney from himself, that Baxter would find that he could be one of the "good guys" and that he wanted to be involved in the fight with Shredder after all.
It was thus quite critical to capture each character correctly while moving the story towards its desired outcome. With each Turtle having such a distinctly different personality, each has his own brand of interaction with Baxter. And with Splinter being a human-turned-animal, he can understand some of Baxter's struggles better than the Turtles can.
Baxter and Donatello
Donatello recognized almost immediately that Baxter needed help from those who could grasp his situation the best, but being leery of most humans and especially of former enemies, he still didn't warm up to Baxter right away. He kept him at arm's length, although at the same time he was willing to have deep and intelligent conversations when Baxter was well enough to process them.
Donatello was the one who found Baxter hurt in the Technodrome right after April did, and it was Donatello who carried him out. He has become much more friendly to Baxter since then, and as fellow scientists, they have that interest in common. With each being a very temperamental sort, they also tend to disagree and argue over how best to create and repair various inventions.
Baxter and Leonardo
Leonardo was kind right away and was among the most willing to give Baxter a chance. At the same time, he recognized that there was possible danger by bringing Baxter to the Lair and worried whether it was the right thing to do. He was soon convinced it was, however, and was often the one trying to convince the other Turtles that they had done the right thing and that Baxter was most likely not a danger to them. He is often a kind voice of reason and listening ear when Baxter needs it.
Baxter and Michelangelo
Michelangelo was definitely the most friendly and forgiving right away, accepting Baxter as soon as he was awake and aware of things. Even in spite of things such as Baxter using the mutation gun on him in the past, he wants to let bygones be bygones. He was the first to really see that Baxter was a changed person and that there was no reason to loathe who he had become, even if what he had been in the past was still detestable. He is usually the one to volunteer to look after Baxter and can sometimes even have deep conversations with him. Baxter sees in Michelangelo shades of the idealistic person he used to be, and he hopes that Michelangelo will never become jaded as he did. It is Michelangelo with whom Baxter has the deepest and closest bond.
Baxter and Raphael
Raphael was the most bitter and suspicious, and of course, not without good reason. It took him a long time to feel much of anything better about Baxter at all, and he still shows that perhaps he doesn't fully trust or understand their new ally. At the same time, he is loyal and protective. He is angry about Barney's behavior, feeling that Barney is completely ungrateful and that Baxter shouldn't waste time and energy worrying about him. He doesn't want Barney to hurt Baxter any more.
Baxter and Splinter
Just as Splinter is the wise teacher of the Turtles, so too he mentors and counsels Baxter. They have shared many important discussions about being a mutant, being human, and how to keep hold of one's humanity even in horrible circumstances. Splinter is very patient, seeing the goodness in Baxter that he long ago was certain he had lost, and tries to guide Baxter towards realizing and accepting that he is still a good person and that there is still hope for him.