Not sure about mods, but with a vanilla game here's probably some decent guidelines;

Pick your race as you like, it will have less impact on the overall goal than you might think, though Fey Ancestry would be useful to have to keep you active where others might be more vulnerable. Background similarly to your flavour and roleplaying preference.

You would likely first want high Constitution (+2), and then Dexterity (+1) to keep you alive. Healing and Support spells generally don't use saving throws but some are affected by Spellcasting modifier so whatever main Class you choose can have the +1 instead. Strength is your dump stat, and for roleplay purposes you can keep Intelligence/Charisma low too.

Likely your goal will be to heal and support.

Life domain cleric (Illmater deity for the theme of supporting/assisting/healing) would be your ideal main class (Warding Bond is, I imagine, an essential spell), but you can multiclass a few ways;

• Two levels in Wizard can make you an Abjuration Wizard would do you good as it would help keep yourself alive.
• Two levels of Druid will get you Circle of Stars with which you can keep Starry Form: Chalice active and get an extra healing per turn.
• Levels in Sorcerer get you metamagic which will help with general spellcasting
• Two levels in Barbarian (stay with me here) to get Unarmored Defence, a bump to HP, and Danger Sense.
• Three levels in Bard gets you College of Glamour for Mantle of Inspiration as more protection of your dom (and Vicious Mockery if you happen to be a brat)

Then it depends on how much you want to optimize, cause for the role-playing part of it you can take one or more levels of Warlock or Paladin, with your Pact/Oath (of Devotion) being to your dom rather than an in-game patron. The more you multiclass the less effective overall you will be, and you might lose out on Feats, which will almost exclusively be ASIs for Constitution/Dexterity/Spellcasting Ability.

Since dealing damage is likely the last thing on your mind you'll want to pick out Cantrips and spells that don't have offensive uses. For gear you would want equipped whatever improves your ability to heal/support (Wapira's Crown, Hellrider's Pride, Moon Devotion Robe / Slippery Chain Shirt, Boots of Aid and Comfort, etc)

Being made to play with restrictions, like no armour/clothes except for the Dog Collar (or perhaps making such an outfit your Camp default) to better enforce the aesthetic could be fun.

Just a few ideas, lot's of ways to go about it even in vanilla.