Quick Answer
Niklas is an item-management Vanguard. His key mechanic is Red Hot Deals: he consumes the left-most item after surviving combat, permanently banks its stats, and converts the item into Shard or economy value. Patch 0.5.3 improved his base usability by giving Red Hot Deals +1 item slot and allowing him back into the Red Rift starting-Hero pool.
How Red Hot Deals Works
Equip Niklas, place the item you are willing to sacrifice in the left-most slot, survive combat, and let him consume it. He keeps the item's stats permanently and opens the slot for another item later. The most important warning is that he does not permanently inherit every special effect from consumed items.
This makes item order a real gameplay decision. Do not accidentally place a premium unique-effect item in the sacrifice slot. Feed him useful raw stats when the team can afford the loss.
Specializations
The Dealer is the intuitive and popular item-creation route. It has Wheel and Deal, direct damage, Max HP, Attack, Magic scaling, Stun, and item creation, but official history says it was weaker than the other two in higher difficulties before buffs.
The Executive is the personal item-slot and Rush Shield tank route. It wants multiple equipped items, Max HP, Rush support, and frontline value. The Hoarder adds Tank identity, Shard scaling, retaliation, Vanguard/Tank support, and Backup value.
Best Stats and Items
Niklas wants Max HP, Attack, Attack Speed, Defense, Magic, Crit, Mana Regen, and Shards depending on route. Good consumed items are usually stat-heavy items, not items whose main value is a unique effect. A fixed best-consume list is 待确认.
Niklas pairs naturally with item generation, including Fiona Right Hand material. He can become a tank, economy engine, or support piece, but only when you manage item order carefully.