ALL heroes are usable REGARDLESS of their IVs.
Now, congratulations, you got the best IVs for Delthea (a good other option would have been +Spd -Def but yours is as good).
Thanks to her weapon, she's particularly useful in a team where everybody else is a meelee hero (https://fireemblem.gamepress.gg/weapon-skills/dark-aura). Thanks to her spread, she can work as a nuke if built and supported correctly and can even tank a few red mages' hits.
Yes, you can switch whenever needed. If you are new and are not yet ready to face high-tier PVP and end-game content then the best advice that I can give you is to try things out. Switch members of your main team with other heroes, like Delthea, and see what works. Depending on the map, she will come in really handy or may be completely useless. Have fun trying to make new team comps that may or may not work.
If you want to use her Dark Aura, the free L.Ike/BlackKnight + good green (e.g Raven/Fae) for starter. You can swap them with stronger unit if you have them (e.g. Hector variant / Halloween Myrrh).
Or if you want to use Blade build, you can get a good buffer for her teammate (e.g. Eirika / Titania)
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