If you have someone that you treat as if he were your brother, this is an example of a relationship would be described as fraternal.
Origin of fraternal
- Middle English from Old French fraternel from Medieval Latin frāternālis from Latin frāternus from frāter brother bhrāter- in Indo-European roots
From American Heritage Dictionary of the English Language, 5th Edition
- From Middle French fraternel, from Medieval Latin frāternālis (“fraternal”), from Latin frāternus (“of or pertaining to a brother, fraternal”), from frāter (“brother”).