What category to file under in EEO Complaint of Discrimination form?

Being an easy target and a quiet loner doesn't mean you were discriminated against. Discrimination doesn't mean different treatment or different impact, it means that the differences are because you're in one of the protected classes: gender, sexual orientation, age, race or national origin, religion, or any other feature of a "protected class."

Unfortunately, bosses can be jerks, and that's legal. What;s not legal is if they're jerks that discriminate because of your gender, sexual orientation, age, etc. etc. If your boss ever made a comment to you or others that you were targeted because of one of those factors, that can be the basis of your claim. If that never happened, them you might not have been discriminated against, as that terms is used in labor law.

Discrimination claims are very fact specific, so you're best off consulting an employment lawyer and disclosing everything you've observed and experienced at your workplace, so your claim can be properly evaluated.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.