Marth

Closest thing Fire Emblem will get to a poster boy.

Marth is a good-looking King from Fire Emblem, where blue hair is always the dominant hair gene, and if it, it's your wife's hair. He's surprisingly not as girly as one might think and can be rather serious when he needs to get shit done. He's also very reliable and can take business into his own hands.

Plot
After thrashing Ike at DDR, Roy manages to get Marth's attention without anyone else noticing.

Marth lightly scolds Roy for being in the mansion in the first place, but Roy begs for help from Marth and puts on a cute act, and of course Marth can't resist that cute face of Roy's so he tags along for the ride.

History
I haven't played the first game but Marth does a thing to resolve something that was going on then he marries his Fiancee Shiida who later leaves him for Tiki.

With his newly divorced life and giving full custody of any kids over to Shiida, he now walks the land as if he hasn't had any tragic backstory or friends for that matter.

That changed when he got into Smash.

He looked after Roy until he had to leave, developed a crush on Ike when he came to smash but is too scared to admit it incase it all goes horribly wrong.

Roy
Marth views Roy as a little brother over everything else. He even treats him like one at most times. And he doesn't believe in FE:F sibling logic so he hasn't fallen for Roy romantically. Marth also finds Roy to be very adorable and huggable.

Ike
Aside from the crush Marth has on him, Ike and Marth seem to be on good, friendly terms. Their time in SSE surley helped that. They also see each other as rivals a little bit, since they like to be better than the other at a certian activity.

Lucina
Despite being her Ancestor, Marth holds very little interest in Lucina and would rather not talk to her or go out of his way to embarass her. In return, Lucina feels somewhat intimidated by this.

Meta Knight
With the time spent in the SSE together, they formed an unlikely friendship. Marth seems to play along with anything Meta Knight wants to do. They are said to regularly practise swordfighting but no one has ever seen them do it.