Should I kill off my Winter Lissa?

I personally wouldn't, Bold Fighter is good but it can be obtained now from non-exclusive heroes. Sacrificing a seasonal hero seems like a bit of a waste... Also Lissa has one the highest BSTs (Base Stat Totals, all stats added) in the game as of this moment.

No. Lissa is the best mixed tank in the armour pool, better than Sheena.

I wouldn’t do it, especially if she’s your only one.

Now that Hardin is in the regular pool, Bold Fighter can be gotten anytime (with luck). So sacrificing a limited seasonal for a good skill isn’t really a good of an idea as it used to be.

Depending on your IVs she can be one of the best mixed bulk units in the game. I have a +Res/-Spd and I can’t wait to give her DC and QR+Bold Fighter. She’s very flexible too with Brave/Slaying Axe sets.

So I’d hold off on giving BF to those units. You may get lucky and summon a terrible IV Hardin down the line.

Never sacrifice the cutie of the season. As they mentioned, Hardin is now in the pool (albeit, super rare to find probably), so your chances of getting BF that way is actually there. :)

Please don’t kill off Lissa..! She’s too good!

I considered doing this when she was the only one with Bold Fighter, but now it wouldn't even cross my mind.
Wait till the next time Hardin is on a banner. I did that with Grima. 3 out of the 4 Grima I pulled turned into VF fodder, and my W! Tharja is safe and sound in my barracks.