General base line is:
Point - Build Meter, be able to utilize assists well
2nd - Build AND use Meter well, Have a good assist and possibly a good DHC, be a good DuoTeam together with the 3rd
Anchor - Use Meter well, don't require assists to function, have a strong assist and possibly a good DHC (if not even a safe one)
Of course these are very basic rules, and even these can be broken no sweat. They just give a decent overall overview of the charaters' main roles.
For example Double:
- Requires Meter to really work, as Catheads is an integral part of her gameplan
- Is bad at Building those Meters for herself, because Catheads is a 2 bar Super = very costly
- Catheads is one of the best DHCs in the game (full combo on hit, safe on block, saves you one bar to DHC into)
- Catheads makes her not need assists
- Hornet Bomber is (one of?) the best assist in the game
She can certainly run point, but she's obviously an easy pick for 3rd.