Marius Phineas Black (1917-1994) was the third child and second son of Cygnus Black II and Violetta Bulstrode. He was the younger brother of Pollux Black and Cassiopeia Black and the elder brother of Dorea Potter, née Black.
Marius had dark hair and tanned skin that matched his broad smile. He wasn't as dashing as his elder brother Pollux.
Marius was passionate and kind, but hated by his family for being a squib.
He was born as the third of four children and born a Squib, without marriage. He was kept in the basement for most of the time and considered a disgrace. His siblings had gone to Hogwarts in 1923, 1926 and 1931. This meant he was mostly alone. He was kicked out of his home in 1933.
He published articles in every magazine he could find about hard lives for squibs. At some point, he married muggle Bertha and had two non-magical children: Carly (1945) and Nathan (1950).