Honestly, Second Characters can be anyone. Either an Alternate Point Character that can help build more meter or can switch with your current point for matchups, or you can use the Second Character to burn the house and swing momentum greatly in your favor to lock up a match.
Point Characters should be able to function well without meter, and can build it rather well.
Anchor characters should have a massive increase in damage when they have Meter and Drive, and should be able to put someone away very quickly in order to comeback if needed, or to finish off a match.
Also to keep in mind, it helps to not have all 3 characters be of the same discipline.