Soleil is my next merge project, I have enough merges to get her up to +10 and my current one is +Atk -Res, but I also have a +Spd -Res, I plan to inherite pretty much every sword onto her so I might try to aim for -Hp since refining the sword will patch up the -Hp
Probably speed. She has 38 neutral attack and 35 neutral speed, and if you're gonna be inheriting practically every sword on her, then speed can be a nice boon for Brave Sword.
She's already got like one of the highest attack stats of the Infantry sword units if I recall.
I went with speed as I initially planned a Brave Sword build for her but it helps with everything, especially Flashing Blade on infernal and abyssal battles as even healers can have massive attack but little is likely to get near her speed.
Image is UB of her actual set but I'm doing this on my laptop in a boring meeting at work. I look busy.
I went with +Atk for similar reasons, and my build is similar as well. I just went with Swordbreaker and Atk Smoke in B and C to help secure Galeforce triggers in more situations.
That said, the reasons stated by others to go with +Spd are totally valid. It all comes down to how you want to build her.
I have a +Atk Soleil with Galeforce and Heavy blade and she's been working out wonderfully for me. I'm still working on the build, but Atk/Spd Link gives her the boost I need to double just about anyone.
I think a +Spd Soleil will be just as good. I decided to go with +Atk since I already had most of the skills needed for the +Atk build.
Pokemon GO
Fire Emblem Heroes
Fate Grand Order
Dragonball Legends
Dragalia Lost
IV Calc
Heroes