Episode 1: A Helping Hand

Make a Perception check with advantage for Rhevia.

Make a Stealth check for Rhevia. How close or far away does she stay from the humanoid while following?

Without trying to seem like I'm dodging the question: she'd follow as close as she must in order to keep him/her under observation but no closer. Skimming the treetops would be my guess. I'd be okk with her flitting/gliding from tree to tree as well. But I wouldn't want her getting close enough for any kind of contact.

For sake of clarity, the reason I ask is to determine a DC for any Perception checks to maintain observation as Rhevia gives chase. Since her darkvision range is 120', I will assume she stays at that distance to stay within your guidelines. Note that it increases the possibility of more obstacles interfering with line of sight, so if you wish to have Rhevia shorten the distance, let me know.

This will also come into play should any of the characters decide to follow. You should also decide what you are going to do.

Whoops. I meant to say, you should all decide what you are going to do.

I understand why you asked. I just felt douchey giving my answer. I didn't know how to describe it otherwise. I just didn't want you to think I was purposefully being a smart ass or difficult.

You read it exactly right. I'd put her at around 100'. As far away as possible without risking losing the target but not so far that she can't clearly keep a good watch.

Thanks.