Just to clarify...
Let's say theoretically, I roll a 5* +res/-atk Delthea, so I'm sad, but even with -atk she's a monster. So I use her anyway, using SI to put all the goodies she needs to be a cute little harmless nuke onto her... then a month later, I roll a +atk/-def Delthea. So I think 'great!' and go to merge them.
Now... I've gathered from several people asking, that all the skills my old theoretical Delthea has - INCLUDING SI skills - will carry over to the new one. But what no one seems to answer is: will the new Delthea already have those available/ready to equip, or just unlocked/able to be earned, and I'll have to grind SP for days for her to re-equip everything?
Theoretically.
As I just recently asked that same question I can confirm, since I just did that with my Kagero.
When you merge you then can go to the equip skills for that character and put those skills on right then and there, no extra SP required.
The only difference is the level of your character.
This is true, but just a point to add onto the last sentence: You also lose SP. I learned this the hard way when I merged bad Delthea and Celicas into better ones I got during Hero Fest... After their Tempest Trials, and I lost of total of 4000+ SP. So just remember to use up all of your SP before merging, if your character isn't fully optimized already, that is.
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