Nino and Jaffar

From a game lore viewpoint, GO FOR IT. Nino and Jaffar were 2 of my favourite characters in FE, so I know the feels.

...That being said, from a gameplay/ utility viewpoint, that really isn't the best idea. Nino with her bladetome is MUCH better off with a support unit (Eirika/ Ephraim are good examples) as those extra buffs are what makes Nino a monster.
Jaffar on the other hand is weird, his kit, whilst niche can be really good with a dancer (hopefully you have a few good ones from the dancing banner), as with poison strike + his dagger, thats an immediate 17 damage + debuffs, and then dance again to ensure a kill either by himself or from other teammates. He can outspeed most units, so unless they have distant counter AND quick riposte, he should at least survive a few encounters.
TLDR: for better utility, Nino + support unit and Jaffar + dancer
for game lore, YES

Honestly, Jaffar is an awful unit. His only remarkable stats are HP and SPD. Everything else is mediocre. I tried various builds on him because he was one of the most memorable units in Blazing Blade...but despite ideal IVs and a legendary dagger I couldn't get anywhere. Kagero and Felicia completely outclass him. In fact, I found my 4* Matthew more useful. So if you really want to use him go for it but expect it to be a challenge.

As for the pairing... One option might be to put Jaffar completely into a support role. Give him Wind/Watersweep, Hone SPD slot, a Fortify seal, Darting Blow in the A slot, and Harsh Command so he can reverse Panic on Nino. He could debuff targets for Nino to finish off.