Reggie has the capacity to also know what she is, and she occasionally feels shame over it. She really does care for people, she just needs help being brought back to reality and being mature while also keeping to who she is. In comparision, Star is also immature, but she takes little to no responsibility for her actions while also doing whatever she wants. She just got worse over time while: bringing down her friends to her level, being irrational in her decisions, and generally being a bad, destructive friend.
Marco's life would be made easier with Reggie and her own as well. Marco's a good guy as well as a fun guy; the fun he and Reggie could have would be enjoyable while helping her to become her own brand of safety kid of a sort. Especially with helping her to realize she has to let things go sometimes, especially with cleaning up her junk and problems. In addition, The Endless would be a good place for Marco to satiate his wanderlust inatead of becoming a bitchy jackass after Mewni.