1. They are both priests.
2. They are not human. (Which means they can change into different forms, fly, teleport, etc., etc.)
3. Gourry is the first to figure that they aren’t human.
4. They have really old blondes as superiors. (But Zelas looks a HECK of a lot better than Saichuro.)
5. They have the same fashion sense. (Cloaks with half-cloaks on top and trim, gloves to the wrist, etc.)
6. They don’t use swords.
7. They like a good cup of tea.
8. They use underhanded means to get their way.
9. They first meet the Slayers when they show up, out of the blue, and cause a bunch of trouble.
10. They have obsessive comic reliefs hanging on them. (Martina and Jillas.)
11. They have the tendency to show up at the very end of episodes, after the mortals have already spent the whole time making fools of themselves. (Usually in interesting costumes.)
12. They don’t mind dressing others in costumes.
13. They usually stand around, not being very helpful, while the others do stuff.
14. They direct the Slayers to various places, missions, and plot developments.
15. They know obscure things that no one else would know.
16. They always figure out or know who is the real “Puppet-master” is before Lina.
17. They both don’t mind doing the whole martyr thing. (Filia for the mistakes of her people. Xellos for what he considers his own miscalculations.)
18. They have spent one episode hiding behind a building with a young, female, human bystander with name consisting of 4-letters, or 2 syllables, (Kira and Anna) while the other Slayers get into a big fight. (Pretty specific, eh?)
19. They both look nice in a dress and they don’t mind showing a little leg.
And finally. . .
20. They both go out of their way to annoy each other.
Thanks BattleJoy Walton!