High elf vampire spellsword Valerys, essential and can be married. Found in Whiterun at the Drunken Huntsman. She has all the strengths (like frost resistance and immunity to poison) of vampires, but also their weaknesses like being weak to fire and weaker in the sunlight. Like vampires in the game she will use one handed weapons, ice and shock destruction spells, raise the dead and use vampiric drain. She will level with you all the way to 300 so she will be useful on all levels.

She comes in two versions, one uses FemaleElfHaughty, a proper high elf voice, but this version REQUIRES Relationship Dialogue Overhaul mod by cloudedtruth which adds follower dialogue for voices otherwise unavailable to followers. The other version uses FemaleSultry voice and can be used without RDO, but really if you are not using RDO you are missing out on a great improvement to your immersion.

Installation:

Extract /meshes, /textures and ValerysVampireFollower.esp to your Data folder and activate the mod.

For RDO version you must have RDO installed, get it here:

http://www.nexusmods.com/skyrimspecialedition/mods/1187/?

Uninstallation:

Delete ValerysVampireFollower.esp and associated files.

Credits:

Cloudedtruth for RDO, without his mod my follower would never have a proper high elf voice.