I got +spd 4*Kagero for first time ,is she good?

+Spd's not the worst, but on a character who's already a shadow compared to her former glory you really need her to be +Atk to have notable worth.

If she's not -atk then by all means, upgrade her because Kagero is amazing. I've used DB3/Vantage 3/Threaten Def 3 on her and she decimates any infantry units.

Since mages are the meta right now and most of them are infantry units, a Kagero is really good. Any 5 star kagero (except a -atk one) will decimate any infantries. You can easily deal with units like ryoma, ike and tharja. I personally do not have a kagero, but her stats seem to be based around being a fast attacker. With +speed, she will double many slow units

I promoted +SPD Kagero a few months ago... I was disappointed by the results initially but slowly I've come around. She isn't as strong as she was pre-inheritance and is of little use against the abundance of cavalry in the meta... She also fails to do much against infantry with inflated stats as seen in modes like Squad Assault and Infernal GHBs. But she still has her uses and does well on an arena defense team.

The core skills I run are Life and Death and Wings of Mercy. I tried Desperation and found I liked teleporting better...especially on a defense team.