Usernames in 1990s: Email local-parts

Legal friction is often low (you are not in a koseki), but platform terms, trademark complaints and leftover childhood handles create a messy, high-volatility system. This page treats handles as a naming system: craft, culture and worldwide platform rules. It will not tell you a lucky handle will make you famous.

Almost never. It can become evidence of a trade name or a trademark if you use it commercially. The default is a login string. Why is practical difficulty so high? Depends on the platform. Some freeze the login forever and only let the display name move — a dual-name system like legal vs lived.

Only as a joke. The relevant inventories are the platform’s namespace and the trademark office if you are a brand. the open point is real-name policies? Some networks have required a “real name,” colliding with chosen names and safety. That is platform law, not Namensrecht — though it can hurt as much.