Besides getting an alternate ending when reaching Lv.40 with Mario, Luigi, and Bowser, you get rewards for reaching Lv.40. These are good items (excluding the Excellent!! Badge's bad effect) so be sure to equip them when you get them.
Intruder Fangs - Allows Bowser to take two actions per turn
Excellent!! Badge - Excellent Strikes boost meter by a lot. If the move was not Excellent result, the meter will be emptied out and you have to start over again. (Luigi's side)
|Intruder Fangs||Bowser Reaches Lv.40|
|Excellent!! Badge||Mario & Luigi Reaches Lv.40|