Learn Portuguese (Portugal): Ele já tinha descoberto o seu superpoder. - He had already discovered his superpower.

This is the compound past perfect. You form it with the imperfect tense of the auxiliary verb ter (here, tinha) plus the past participle of the main verb (descoberto). It translates directly to "had discovered" and describes a past action that happened before another past action.

Could I put já between tinha and descoberto, like we do in English?

No. In Portuguese, adverbs like já (already) are usually placed before the auxiliary verb (tinha), whereas in English they often split the auxiliary and the past participle ("had already discovered").

Why do we need the article o before seu superpoder?

In European Portuguese, it is standard practice to place a definite article (o, a, os, as) before a possessive word. You will almost always see o seu instead of just seu.