Should I invest Ina MR raiyruu or dismiss him 100percent

Rairyuu doesn't hit hard enough but he does have a kind of playful variety with thief subjob and his statuses that make him useful for some of the event farplane quests but he's also highly vulnerable, not a unit to use on auto

if you build an MR build tanks and mage/healers because they always have a use, except whisper

Go all in.

As a newer player aim for a variety of lvl 79 units, eventually trying to get a platform of 3 different element tanks, two different element healers, and 3 different element dps type units... don't worry about LB5 because he doesn't hold up in endgame, but he's worth building earlier on

Good for PvE

Depends on what you are focusing on e.g the arena, guild wars, PVE contents. I would say he would not be usable on guild wars and very difficult on arena but many useful for tower contents etc. If you have other alternatives probably invest in other UR units.