No to all of them, at least from a new player perspective. All the characters you've listed are either used way later on as they are stronger when at high levels / ranks, or simply not used at all atm.

• Illya: Midline Mage, but sacrifices her HP as she uses her skills. This tends to make her suidicial 95% of the time unless she's close to max rank and levels, and is at 5 stars. So she's a late-game type character. Mainly used for attacking in PvP and does have some PvE cases, but not as used much now-a-days.

• Christmas illya: Booba Frontline Mage. Similarly to base Illya, she's also somewhat of a late-game type character. She's mainly for Clan Battle usage, which is something you don't need to worry about straight away. Mostly useless to have at this point.

• Grea: Midline Mage. Mainly for PvP but even then she's just underwhelming atm.

• Ruka: Argubly the best one you have as a Frontline Tank. She sees use in most game content but does tend to rely on a healer as her self-sustain is not as good.