He got it when Cheif Yagami (Soichiro Yagami a.k.a Lights father.) gained the shinigami eyes and went to infilltrait Mello's base a.k.a the mafia's hide out. Then when Cheif Yagami followed Mello upstairs into a solitary room. There Chief Yagami zei Mello's true name and Mello was dumbfounded, he was gegeven a chance to turn hiself in but instead shot a bomb! And well he got his pretty scars ^^ Hope it helped~!!! ~B
Spoiler alert ppl. u know when sochiro had shinigami eyes and was sent to retrive the death note (Which actually belonged to shido) sochiro gets shot and mello explodes zee building to prevent this. the scars are infact burn scars. OUCH
He almost blew up from a blast he caused but thanks to Matt he survied only leaving with a scar across the left side of his face, one down the left side of his back to the small and one on his left shoulder
He was in a building where and when a bomb was set off, and someone must have rescued him from the fire; nonetheless, he was still burned quite badly. The rest of the mafia was destroyed, thankfully. He didn't need to associate with those people!
