Bite-Size was a Little Brown Bat that the Wild Kratts met in "A Bat in the Brownies".
He was flying around the forest and the Tortuga HQ, catching bugs, until he landed and hid in Jimmy Z's brownie dish. Once he was found by the crew, they all got scared of him, except for the Kratt brothers.
Jimmy and Aviva were able to rethink their concept about bats, Aviva even went so far as to call Bite-Size 'delicious' (which meant she thought he was cute) but Koki kept being stubborn and mean about him (even expelling him from the HQ in the first opportunity).
What made her change her mind was seeing Bite-Size and his bat friends homeless after lighting hit the tree they were living in. After the Wild Kratts sent a bat box plan for every Wild Kratts kid around, his friends found a lair quickly.
Bite-Size choose to return to the Tortuga HQ to live in Koki's bat box (it is understood that this was her way of apologizing for her mean actions, and this would technically mean that he forgave her).