the andalite from #40 i think. he had soola's disease, a fatal andalite disease which has no cure, except one can save themselves by morphing and staying in that body, becoming a nothlit.
I don't see why he didn't just either morph a human and stay as that, or simply morph mertil, his friend he was taking care off. they could of went on to live together long and happily, instead of just dying months after saving mertil from the yeerks. he should of done it for mertil. after all, there were both still stranded on earth, and after ganifilan goes, it would be lonely for mertil. though i guess, after #54, when andalites and earth are open, mertil would have gone back home.