One of the main problems of Dark Dawn, is that the game duration does not justify at all 8 characters. If we think about the importance of each character:
Venus: Matthew. Because he is the son of Isaac.
Jupiter: Sveta. Because she is actually the real main protagonist of Dark Dawn, even if you play with Matthew prespective, the plot of Dark Dawn goes around her race and city. And she has various key roles through the history.
Mercury: Amiti. Because he is the son of Alex. Comparing to the rest of the characters his personality is interesting, i really enjoyed his "we are not going to help that dirty pirate" attitude.
By simply adding a Mars Adept, that can be any of the remaining 5 characters, we are all set.
But well lets consider the characters:
Eoleo: Well, this character was not executed correctly for just one thing, but that thing is enough to ruin the whole character. He joins our party for two reasons 1) The Ship 2) Revenge. And at the end... I would not call that a legit revenge. He does not even have a notorious dialogue before the final battle. This would be relatively easy to fix and this character fits with the 3 characters above.
Tyrrel: This character is the trigger of one of the dumbest and worst managed plots that i know. Notorious for basically being angry, impulsive and a bit insensitive. And for discussing with Karis. Dispensable, we could either find a better trigger for the history or just add him as a secondary dumb character. For example, he could not accompany Matthew due his wounds after colliding with the Soarwing. Or Matthew could just decide to reject the help of "his friend" and go alone,
whatever.
Karis: The main problem that i have with this character is that the first thing it comes to my mind of her are discussions with Tyrrel. I do remember of Dark Dawn that he discusses a lot with Tyrrel. She sometimes appears to be as a "wise" character, but she is most notorious for discussing for Tyrrel. With Tyrrel being dispensable, Karis is dispensable.
Rief: He is wise and smart (of course, he wears glasses). He does have a few of notorious scenes around Alex and Amiti, due being familiar of those. This character is perhaps the best between all the descendents of the first game, unfortunelly. The mercury spot has been taken already, And even if he has importance in the development of Amiti as a character, that can be easily changed.
Himi: I would say that she is shy at minimum. She barely talks, the excepction mostly occur when she is in "trance". In that state she does not act like in the rest of the game, her personality changes. Actually i do remember a scene in where all the characters say their respective line presenting to a specific character, with the exception of Matthew and Himi, that they dont talk. Someone presents Matthew, but Himi is ignored the whole scene, just staying there without getting involved at all in the conversation.
At the end i would have Matthew, Eoleo, Sveta and Amiti as characters. The other 4 characters are not that bad, but you dont need them as well.