To look someone up, no — enter any RetroAchievements username and you can see their mosaic or build a card from it without an account of your own. To have masteries of your own to show, yes: achievements are earned through RetroAchievements, so you need an account there and games you have mastered. Either way you never sign in to MasteryHub and no password is ever entered.
Yes.
A community project that adds achievements to retro games. You play through an emulator that talks to their servers, and unlocking achievements works much like it does on a modern console. It is free, run by volunteers, and covers thousands of games across dozens of systems. See retroachievements.org for more.
Every achievement in a set, earned in hardcore mode. Only Mastered games appear on MasteryHub.
Results are cached for a short while so the site is not re-fetching the same profile on every visit. Wait a while and refresh the page to pull a fresh copy.
A single-game version of the mosaic. One card built around one mastered game, with its own settings for the header, icon glow, platform, earned date and card styling. It opens on your most recent mastery, and any other one can be picked from the dropdown above it.
Only in your own browser's local storage. Nothing you customise is sent to or kept on the server — a different browser or a cleared site data will start from the defaults again.
Reddit (u/sanshigoo) or Instagram (gg.sanshi) get the fastest reply. contact@ramasteryhub.com also reaches me.