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It depends on why they're using a different name.
Oliver Horn sounds like a real name being concealed and then dramatically revealed, which is Given Name Reveal.
I am not the least bit familiar with Motoko Kusanagi. If they are some kind of fighter, maybe it's a Nom de Guerre.
There are several other options such as Red Baron (pseudonym given based on awesome skills), Pen Name (used by a writer or artist), Moustache de Plume (used to conceal gender). I'm getting most of these from the Naming Conventions index.
Edited by KD
Only Known by Their Nickname's description excludes characters who are only known by an explicit pseudonym. So what trope should I use instead?
Thinking of characters like Motoko Kusanagi (said to be a pseudonym in several continuities) and Oliver Horn (concealing his real surname for Myth Arc reasons: his parents' surnames are Groves and Halford).