You should probably bold questions when you ask me, otherwise I may miss them, but:
Blue and Rouge are both mages from the Magic Kingdom who are tasked with mastering all of the magic in the various Realms.
Blue has the specific mission of mastering these magics and killing his brother, Rouge.
Since you don't play as Rouge, it's unclear if Rouge also has the goal of killing Blue, but it can be assumed.
Most other players can recruit Rouge, who is probably the best human character in the game due to his Gift for Realm magic and his great stat growth.
No player can recruit Blue. Red gets the option, but when he explains that his name is Red, Blue kicks him out of his room (because Red is similar to Rouge, get it?). Blue is a bit of a cynical character, who only sees others as valuable tools rather than actual allies. Rouge, on the other hand, seems to lack the aggression and cynicism that Blue has.
When you play Blue's story, you must travel from realm to realm to learn various 'Gifts' for different schools of magic.
Every school of magic has an opposed school of magic and you can never learn the Gift for both. They are;
Realm and Mystic
Shadow and Light
Arcane and Rune
Time and Space
There are other magical schools that exist, such as Mind, Evil, Mirage, and Life, but these do not matter for Blue's quest.
Every time Blue learns the Gift for one type of Magic, Rouge will eventually learn the opposing type. So if you decide to learn Shadow Magic in the game, Rouge will decide to learn Light Magic. Once you have the Gift for Realm, Shadow/Light, Arcane/Rune, and Time/Space, you duel Rouge to the death. Upon winning, Blue absorbs Rouge and is capable of having the Gift for every type of magic. If you lose, you continue play as Rouge.
Rouge does not get Shadow Magic initially. Both Rouge and Blue start with Realm magic and can travel about to obtain the other 'Gifts'.
In SaGa Frontier, you can cast magic from a school if you don't have the Gift, but you need the Gift to learn and cast the more powerful spells.
If you ever get the Gift for one school, you have to give up all magic you have from its opposing school.
Mecs and Monsters can't learn magic, but Mystics and Humans can.
Rabbit is probably the strongest Mec character in the game (besides T260G, of course). Rabbit has unique access to a Bit system, which fires little
satellite bit things that float around and shoot his enemies with lasers. His move, SatelliteLinker, is one of the more powerful moves that a Mec can get.
Rabbit only joins Red's storyline, as it turns out that Rabbit is an agent of IRPO searching for the Black X drug factory in Kyo. It's unfortunate that more characters can't recruit Rabbit, because he's very powerful and his SatelliteLinker ability can make quick work of bosses.
A roleblock would not affect Silence's role. Commuting happens before every action in NAR except for role-copying.
