Grandia is an unusual RPG in that it features a fairly complex system of building up characters and at the same time has multiple characters that leave the party permanently. Normally, it is very disappointing and disheartening when a character you have spent hours building up properly leaves, but Grandia has an interesting mechanic that helps alleviate this problem.
Grandia's ability system is built around building up a characters skill levels in their weapons and in the game's four magic elements. By building these skill levels up to certain pre-determined levels, a character can learn new special moves or magic spells. However, doing so can involve putting in a lot of dedicated effort and intentionally dragging out fights to acquire more skill experience points. Furthermore, giving a character access to one of the four magical elements requires the player to trade in a Mana Egg, a rather rare and valuable item. So, building up a character requires the expenditure of a lot of time and limited resources.
Normally, it is advantageous for a player in an RPG to neglect spending valuable resources on temporary characters. However, even knowing beforehand that certain characters in Grandia were going to leave the party at the end of the first disc, I still ended up building them up. I felt comfortable doing so because Grandia gives a consolation prize of sorts when a party member leaves for good: skill books that can be used to transfer part a fraction of the old character's skill levels to another character. So, if I spend a lot of effort building up the stats of a party member who will leave, I can give a part of those stats to either help a new party member catch up or to help a character get a high level move late in the game. In that way, building up a character who is going to leave becomes advantageous in the long-term, since if you don't spend the time building that character up in the first place, you won't have any skill levels to pass on to other characters.
Grandia II does something very similar to the original in this regard, but at an even better deal. When one of the game's characters leaves the party for good, the player gets a skill book that transfers that character's accumulated skill points and magic points in total to another character of the player's choice to do with as the player pleases. I think this approach to handling leaving party members is a lot more interesting than what happens in games like Final Fantasy V, where the party member who leaves is replaced by a carbon copy clone stat-wise.
There is a lot of story potential to be had from a main character leaving the party, and it would be a shame to let a game's ability system get in the way of that. However, you don't want the player getting too upset about a character leaving for game mechanic reasons, and giving the player a reward for putting time and resources into a temporary character is a great way to ward that problem off. It encourages the player to become more invested in every character, irregardless of how long the character is in the party.