1st division commander of the Whitebeard Pirates. Also known as "Marco the Phoenix." Endowed with the powers of a Devil Fruit, he is able to transform into a phoenix. In his phoenix form, he can withstand any attack and regenerate any wounds.
Short version: He is the only character with a super-useful full-heal special. Useful captain too, and good stats: TOP TIER CHARACTER.
Marco is an extremely useful character, easily rivaling his legendary version (at least a sub). His full heal makes tank teams twice as durable, and when you add Moby Dick, it effectively triples the tank, effectively in the range of ~100k HP, give or take 10-20k depending on team combination. Of course you will want to max out his special, doh.
Socketing guide: 4 sockets means chose 4 out of 5 good ones (Anti-Lock, Anti-Despair, Auto-Heal, Matching Orbs, Charge Specials). No perfect choice, either chose your favorites or let RNG help you. Keep in mind you don't want Anti-Despair on most Blackbeard teams...
He sees some use as a captain, too. A PSY lead against INT turtles, that's one, and sometimes, he is used as a lead for high damage PSY teams against INT bosses (ex. in Clash!! Zephyr!). Risky, but can work.