Yes, most people there are usually nice, or at least don’t give you any trouble. But here’s few examples who I’ve met:
They are really harmless. They come and go as they are searching for rare island items. They rarely say anything at all, so if you’re wondering why everybody leaves immediately, it’s not because of you.
They will trap you on the island. They’ll start a tour and don’t sit down, or they’ll open their menu when you try to leave. They usually want bells. If they stop you by opening their menu, you can start a tour and go there alone. Just make sure you sit down first, you don’t want them to go with you (but if they don’t close their menu at all, you’ll be stuck on your chair).
If nothing else helps, and you don’t want to waste time (or bells) with them, the wifi-switch on your 3DS is your friend. But you will lose all the items you haven’t put to the box before they arrived.
May be awkward. You don’t know how old the other player is, even if they are just playing…
Not as harmful as a captor. Asks how many bells you have, tells you how poor they are, and asks you to give them bells or to buy items. They are a bit lazy, because usually the amount they are asking for is really easy to archieve by catching butterflies or beetles.
Tries to cheat you to give them bells or items, or wants to go on a tour. Sits down on a chair, but stands up after you have sat down, sending you on the tour alone (sometimes it may be just an accident, though.). Cuts the connection on purpose, they will usually comment something suspicious before they do so.