I need a green unit to pair with my +10 Hana

What build does Hana have? I could give a suggestion based on what build you have for her. :)

I guess it depends on your build, but Hana doesn't really have the most defense or resistance out there, so she's probably going to have to hide behind someone that can take some hits.

Camilla is a good res tank, but her defense isn't that high unless you're actually building her up to be a physical tank.

Cherche can be another physical tank, but she has pretty low speed, and most people just build her with Brave Axe to be a physical nuke.

Cecilia is a fairly okay res tank which is made even better with Fortify Cavalry, but most people tend to use her as either a Gronnraven or Gronnblade mage.

Boey is an excellent physical tank, but his resistance is practically non-existent, and if given Close Counter, can be used to counter melee units with Bonfire/Ignis.

Titania has decent resistance and "okay" defense, but she would need Fortify Cavalry (or Res and/or Def Tactic) like Cecilia.

Out of the units, I'd recommend either Camilla or Boey. Camilla can be a good physical and decent magic tank if built right, and she can (if you want her to) carry Hana with Guidance for more mobility.

Boey can make use of the free Close Counter we're getting (if you want to use it on him) to counter with high-damaging Bonfire/Ignis specials to protect Hana while she's behind him, but he doesn't have that much resistance or speed.

Depends on what role you want for the team.

If you need someone to straight up protect Hana from blue units, Cherche is a physical tank and absolute beast whether you keep hammer on her or give her brave axe, but mages/clerics will down her easily.

Camilla in my eyes is a mixed tank, just really expensive to build. She's a really balanced unit that is worth it in the long run and can be flexible in her role on the team, but she takes some dedication to build.

Cecilia would be a good choice for magic overage and depending on whether you give her the bladetome/L&D/Desperation build or leave the raven on her, she can be really aggressive player phase unit or a traditional mage tank and check archers that may target Hana's perhaps weak defenses.

Titania I don't know too much about, other than she has great res but is relatively weak. She can definitely guard Hana from the likes of Reinhardt and Linde, especially if you give her DC. She can keep blue mages at bay easily with her given axe. Some go for poleaxe though, and make her anti-cavalry.

Boey I don't think would be as helpful as the others, simply because he'd only be a shield and the owltome doesn't mesh will with Hana's playstyle. Boey can run the raven build better than cecilia if archers are a serious problem, but other than that, I don't see him and Hana on the same team.

Firstly, wait for Camilla new weapon, it might be good and even without it she is a great unit by herself.
Your Hana seems great in all aspects, maybe you need a good brave lyn + reinhardt check so Cecilia with raventome triangle adept is a good idea. Most importantly, why don't you favourite your Hana?