By "optional" I mean "you can complete the game without hiring".
In Mass Effect 2, you could finish the game without hiring Kasumi Goto (a DLC character), and I think Grunt and Legion are optional as well (you have the option of not animating/reactivating them), though all others were mandatory.
What about Mass Effect 3? Is it possible to finish the game without hiring one of the characters? (I'm guessing probably not, but I'd like to be safe). I'm not talking specifically about DLC characters, but any character that is possible to miss.
Best Answer
Javik the Prothean is a DLC only squad member that's available in the "From Ashes" DLC:
And as @LessPop mentioned, the events in previous Mass Effect games will mean that there will be some squad members that are completely unavailable in the game:
Other than that, the only way to 'miss' a squad member is to follow certain decisions in ME3 which results in their death. There's no way to avoid recruiting any of the non-DLC squad members in ME3, they all show up as a part of the main quest.