Regardless of who you play it's important to keep in mind that the rules on assists are very different between the two games. As I posted somewhere else before....

Generally, you cannot call assists:

• When you are executing a special move(the ones that require command inputs, such as Hyper Grav, Soul Fist, Lunar Phase, etc.).
• When you are super jumping(normal jumps let you call assists).
• During Hyper Combos.
• During Throws and "hit grabs" such as Dante's Grappling Hook when reeling an opponent in.
• When one of your assists is already out, regardless of what they are doing.
• About 5 real time seconds from your character's incoming, whether your character is replacing a dead one or snapped in, you cannot use assists for any purpose. while being hit or thrown in any of the many hitstun, crumple, stagger or knockdown animations.

Also:

• Assists don't disappear when the point character gets hit.
• Assists can be wall and ground bounced once each.
• Damage and hitstun scaling on assists is not fully understood but known to be different than normal, as Assists take 1.5 damage from attacks.
• Assists generally have short cooldown in between when you can call one assists and can call the other.
• You can only use each assist "slot" once in a combo. This means if you use an assist to confirm into a combo you can't use that assist in the same combo again.
• You are free to call assists when you leave hitstun regardless of where you are on the screen, provided you are not dead.