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Mega Spoilers ahead don't read if you don't want to know!!!

When I reach Maini I almost always choose Siobhan. She doesn't say much, but she can use some of the best equipment that no one else can. When she is all decked out with the best gear, she is probably the strongest character in the game.

SPOILERS (You've been warned):
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Khunag is an essential character for the later quest through the Kengent Kamulos
underground city/dungeon and the game wont let you go there without him. It's not essential to pick him up as soon as you arrive at Maini for the first time though.
Anytime after Melthas and Sira leave at the end of Maini you can go back and pick up Khunag and have him in your party as well as Siobhan. I often go to the Kengent Kamulos city with the following party: Tom, Drirr, Harriet, Siobhan and Khunag. Yes you miss out on the powerful spells of Melthas and Sira, and yes you will only have 5 members, but you get Siobhan. That way your party is much less reliant on magic and you can deal with the monsters that have a high magic resistance.
Afterwards you can ditch one of them for the final mission to the Toronto and take Melthas and Sira or you can keep them both and pick up Joe for the 6th spot.