Sync is the reason I left Reddit and joined Lemmy!
That said, the new pricing options scared me away (you can't tag users unless you get the super expensive one? Why? I used to be able to on the Reddit app) so I've been using Jerboa. It's not as good as Sync (maybe 80% as good, for my purposes at least), but it's free and getting better all the time.
You could add a little tag beside their username saying whatever you wanted it to. It was great if you found someone to be particularly helpful or knowledgeable about something and then would see them pop up in other places. I remember tagging that guy who really knew vacuums for e.g.. Or maybe there's some other little quirk. I've forgotten the username of the dude who didn't want to shit for three days here on Lemmy and definitely would've tagged them as such.
Now that I think about it, I remember seeing how many times I'd upvoted people beside their name too (was that in the app or an RES thing?) which is another nice little feature.
For example, I had a tag labeled "don't click" for the guy on reddit that had that weird spinning Pikachu hyperlinked on the period at the end of his posts. That tag saved me from wasting so much time.