Hi I`m playing a Level 15 mage/ level 9 kensai as my main character in multiplayer mode my
stats are 18 18 18 18 15 3 and when i try to cast a familiar it always says `this spell is only useable by (something)` is there any way for me to cast a familiar?

You can only summon a fammilier with a fullmage and a beast
master as they are called protagnists.

That's interesting ... because I am currently (3rd time through BG2) playing a Skald and I was able to get a familiar. I thought it was interesting that my Skald can cast spells. Nice surprise that! So far, this Skald is my favorite character, with the ability to fight and cast spells and identify with no spell necessary!! (For those of you who don't know, a Skald is a type of Bard.)

Maybe it's only the single classes who can
find famillier !Protagnists are singleclasscharacters I think

Find familiar can only be used by the "protagonist" (main character)........