I see your issue: you're visualising a whole bunch of blank profiles entering the site, and other people seeing those blank profiles and going "oh, I'll leave mine blank too".
We can give these guest character profiles a different appearance then: the name and PID in the top block, then none of the other trait/interest/idea/description blocks. Instead, a block that says this profile was used to quick-join a roleplay as an offsite guest, and hasn't actually been properly created yet. (In some better wording than that.) They should also probably not be featured in search until they've properly registered and created their character fully, which we can prompt them to do during registration. ("Welcome to the site! Now, if you're ready, we should finish creating that character you made.")
A case could be made for quickly picking a couple of traits at the same time, but I don't think we should press them to fill out anything else like a blurb. (If we do have htem pick out traits, we should still do the above. ^) This character is for the player and their partner (or partners), rapidly joining a roleplay, not for us. If any of us have an indignant response at them not properly creating their character for our sakes, that's our problem, because it wasn't for our sakes. People can already create a blank character if they want to jump into something quick with a friend as well — many such characters exist, and I've had one of these myself. They may fill it out just because they want to later even if we don't prompt them (but we should prompt them) — I don't like having a blank profile for my characters, they might decide they want a pretty picture like other people, and add more details while they're at it.
If we're creating minimum barrier to entry for them leaping into a roleplay, that does a service to them and whoever invited them. Extra requirements going to have to be worth it for them and their partners (who already know them), rather than to us, or it defeats the point. At the same time, they can go and explore the site, sample what it's like, and sign up properly when they're ready, so offering this also has the benefit of getting more people into Litphoria and increasing site activity. Consider this invitation process to also be effectively a recruitment process, which benefits all of us.