Unit Analysis by Team BethelFrankel
Class
Double Chainer with insane in-element synergies.
Overall
A-
(S+ at BT2)
Advantages
+Double chaining offers the highest possible damage output in the game in scenarios that require burst
+Has some of the best in-element synergies in the game
Disadvantages
-Double chaining cuts attack, which causes it to scale worse than it seems as the game goes on.
-Extremely dependent on ally conversion, which is often out of sync with his CD
-Long CD and lack of preemptive is awful without BT2
Explanation
Double chaining is an extremely powerful effect, but it can often be a lot of trouble for your team due to its slot-hungry nature. This puts double chainers in a bit of a weird spot, perhaps akin to that of healers: in the right situations, they're absolutely necessary for pushing through OTK damage checks, but in the wrong ones, they're a wasted team slot. This is also why Double Chainers are possibly the most BT hungry 5*s after cross converters; at BT2 onwards, their damage with only one other DPS is enough that you can run them as a secondary DPS, which offers more team-building flexibility and less in-game liability.
Istvan is arguably the weakest double chainer because of his extremely niche equipment and meh chain, but he has several relevant synergies with units like Sinsa, Victoria, Uriel, Eicy, Matthieu, the list goes on. As with cross converters, the kit is largely irrelevant - the two or three turns he pumps your DPS through the roof are not.