Zelgius doesn't seem worth my orbs to summon seeing as you already got black knight for free,What do yoy think?

It depends on u but imo I would do it because in terms of stats he is more on the offensive side as well as you can test ur luck and maybe get a +spd or +atk iv which would make him more viable, Black Knight loses in terms of offensive power but in terms of def he wins slightly Zelgius also has a higher bst but then again it all depends on u

Zelgius is good for arena scoring. So if you dont need him to do that, skipping is a good idea.

Well he's handsome so worth it to me ;D i got +spd -hp so he's been much better, and warp powders very useful in armour and defense teams (dazzling staff healers can safely attack then he swoops in and bam free defense wins~)
But it's not like you miss out on much without him so it depends how badly you need other units
I use him alongside black knight, black lunas for all

Well, Zelgius with his base kit, with the addition only of a Dual Rally and Close Defense seal commands about 712 points in the arena. That's huge for a no merge unit.

In my opinion, Zelgius isn't worth the orbs. I have a Black Knight and I hardly ever use him aside from Arena Assault and the monthly "Complete 'x' with Armor only units" and things of the like. If you want him, then cool. He's a better Black Knight, but for someone like me, he's simply not worth it.

Plus his artwork seems lazily done.

Zelgius is good and worth trying to pull but if you already have Black Knight, don't worry about it then because if you miss Zelgius you aren't really missing out on anything too big as they're both definitely not the same character. Zelgius just has a better defensive spread while having a boosted BST similar to the winter units.

While they're both sword armor units, Zelgius has slightly better base attack, defense, and resistance, and his stats aren't set as neutral. His kit is different, with Fierce Stance 3 giving him a more offensive role and Warp Powder giving him better unit. Overall, even if he was a clone I'd say it'd be worth it to have another unit as good as BK.

Pretty much everything you said.