Mana-chan

Mana-chan is Chance's imaginary friend. He invented her when he was still little and didn't have any real friends yet.

Chance
Chance is the only character who seems to be able to "see" her (except possibly for Yuuna). It seems clear that he cares a lot about her. She is almost certainly younger than him, since he refers to her as "Mana-chan".

Yuuna
Yuuna takes a great liking to Mana, though it is unclear whether she can actually see her or is just playing along with Chance's imagination. She is the one who convinced the main characters to let Mana be a part of the team.

Kitti
Kitti once said Chance was "far too old to be making up friends, especially when he already has plenty". This seems to imply that she thinks he should stop believing in Mana. However, other interpretations are also possible: fans have occasionally interpreted this sentence as a hint that Mana might not be imaginary after all.

Washa
Washa finds it incredibly annoying that Chance has an imaginary friend, and he complains about this a lot. Naoki used this hostility towards Mana as an excuse so that Chance would agree on doing nasty things to Washa, claiming the latter had hurt her and they needed to take revenge.