-Both fail to be in a stable romance with someone
-Both use wizard hats (pink hat for Roxy and blue hat for Eridan)
-Both have a pretty low self-esteem (Roxy fills that by drinking, Eridan destroys his own Hope)
-Both use Riflekind
-Both use a second Strife Specibus other than Riflekind (Fistkind for Roxy and Wandkind for Eridan)
-Both have their rifles powered by the deadlier sciences
-Both wear scarves with stripes (blue stripes for Eridan, orchid stripes for Roxy)
Now how about Aranea and Calliope ?