Mihael Keehl (Mello) was born on December 13th 1989 in Los Angeles, California, USA.

Mihael haven’t had the best childhood a child could wish for. His parents were quite poor, his father worked at a bar and his mother worked as a waitress at the local diner.
His mother would work from 9 till 5 and his father from 5 till 12, Both parents weren’t together at home during the day, only around the time they went to bed and in the weekends.

Mihael’s father didn’t quite love his child, he actually said once that he hated him, and he always did. Mihael’s mother was very fond of the little blond boy and gave him as much attention as she could. Every day when she came from work she would bring the boy a chocolate bar because she knew he was crazy about those (this is one of the reasons Mello is addicted to chocolate, it’s the only memory of his mother). His father became very jealous over the fact that his beloved wife cared more about her son then she cared about her husband.
Mihael’s father was an alcoholic and abused Mihael, he hit him and locked him up in his room, he thought Mihael was a little spoiled brat and he needed to get punished for it.

Back then Mihael was still a happy and active kid. Always friendly and polite, though Mihael’s parent’s didn’t had money enough to send him to school and he didn’t really had the chance to make friends his age.

When Mihael was at the age of 7 his mom died. There was a robbery at the diner where she was working, and while trying to get away his mother got shot. Mihaels word collapsed and so did his father. Mihael became a less happy and active, he didn’t smile that much anymore neither did he talk much. His father’s drinking problems became even worse and so was the abuse. At the age of 9 Mihaels father died of alcohol poisoning. From that day Mihael’s personality changed, he became a mean, quiet and unhappy kid.

He got send to a regular orphanage where he didn’t seem to bond with any of the children, he changed his name to Mello, not wanting to remind himself of his rotten childhood. He finally went to school and started to learn things, though they soon enough found out Mihael wasn’t like any regular kid. His IQ was very high, and not just normally high, this kid was a genius.
At the age of 10 Mihael got a special visit from an English man who owned an orphanage in Winchester for extremely intelligent kids and Mihael got transferred to England, to the special Orphanage “The Wammy’s House”. He started a new life there under the name “Mello”, he studied hard in order to become L (the worlds greatest detective) his successor for when he has passed away. Though Mello had allot of competition he didn’t gave up and was already ranked as 2th to succeed L. He didn’t allow anyone to get close to him, afraid that someone or something would distract him and he wouldn’t be able to beat Near, the number 1 student at Wammy’s.

One year later another new kid from America appeared in the special orphanage, his name “Matt”, nobody ever knew his real name but himself. A strange redhaired boy who wore goggles all day. He became Mello’s roommate. Mello didn’t wanted to talk to him or anything at the first place, but after a while they started talking, hanging out together and they became best friends. Matt was very good with hacking computer and not just that, everything that was just a little bit like a computer.. Matt could hack it, Matt could break into it and get passed every kind of security.

At the age of 14 (almost 15 that time) L died. He hadn’t chosen his successor yet and both Mello and Near were called up to Rogers office. He suggested that both Mello and Near would work together and both succeed L. Though Mello refused, yet again he couldn’t win and he didn’t see why he would stay at Wammy’s. He told Roger he was leaving, he took his stuff and left to live on the streets.
On some way he managed to get back to L.A. and life there, in old buildings somewhere in the hood of L.A. At one moment, while fighting in the streets, he got picked up by an adult man called “Rod Ross” calling himself “DWhite”. He was the leader of a local Mafia gang. He took Mello under his wing, telling him he saw a potential mafia member in him. He taught Mello how to use a gun and everything he needed to know. At the age of 20 Mello became, together with Rod, the leader of the Mafia gang , And they got their newest member, one who didn’t really got involved with the business at the hideout, one who just sat in Mello´s apartment all day, working in the shadows. Mello´s best friend and hacker, Matt. Matt had ran away from Wammy just like Mello did, though Matt didn´t go all by himself. He left after Mello called him, telling him he needs him and his talents, and asking him if he wanted to join and live with Mello. After that happened they started working on the biggest case ever,the Kira case. Even after all these years, Mello tried to surpass Near by beating Kira first. Acting like his real intenstions were to get the killing notebook

— This is my own headcanon for Mello. His real childhood was never reveiled in the anime nor manga.