People who have a good singing voice but won't show it.

Metal - He's good in all types of singing, but hates to let it slip out. Will occasionally sing a note or two for a joke, like "DAISY ME ROLLIN', THEY HATIN'!!!"
Titus - Get a crowbar if you wanna hear that voice of his, he's not going to sing against his will, so that means he might never sing. He finds it very embarrassing, so you probably wanna put someone's life on the line or coax (a lot)

People who sing but aren't the best.

Shard - Will rap when he wants to and whatnot, he doesn't care, even if he is off-key.
Sonic - Well duh.

Good singers and are not ashamed to show it!

Heathers!Metal - He will sing for his sweet Heathers!Star, he's just a sweet lovesick teen.
Cyber - He's best as songs by Owl City, Fireflies is his favorite, also can dance! (He will grow up as a singer then change his profession to scientist later on when he's around his mid-twenties.)
Immortal - Her singing voice is Laura Brehm's.
End/Todd - He does sing when he wants to, just not too often.