The My Hero Academia anime is almost like an ensemble cast series. Izuku Midoriya may be the main protagonist, but many of his classmates get their fair share in the spotlight. He's one of 20 students in his class at U.A. and some of them began the series much stronger than others. One of them is Fumikage Tokoyami, the quiet, raven-headed boy with a shadowy Quirk.
Arguably, Katsuki Bakugo and Shoto Todoroki are neck and neck for #1, but Fumikage isn't far behind, largely due to his outstanding Quirk and his mature, practical attitude on the battlefield. In fact, the series could have made him a cheap powerhouse for the sheer sake of giving Izuku a rival to beat (other than Bakugo), but thankfully there's more to him than that.
What Fumikage Tokoyami Can Really Do
Fumikage is one of many characters who rely mainly on their Quirk during combat, in the same league as Momo and her Creation Quirk, Denki and his electric Quirk, and so on. In fact, some fan theories hold that Dark Shadow is Fumikage's long-lost twin, embedded in Fumikage's body since birth and fusing with his own Quirk (while Fumikage's Quirk is simply his raven head). In any case, Dark Shadow has all other Quirks beat by having a mind of its own, being able to provide tactical input and insights, and Fumikage can give it orders. Fumikage is a tag team all on his own and few, if any, other students or heroes can pull that off.
In combat, Dark Shadow boasts incredible speed and agility, being able to outmaneuver even Mina Ashido, who proved to be a rather agile and nimble fighter herself. Dark Shadow also hits incredibly hard up close with its claws and headbutts, and Fumikage has enough control to deal only light blows if he so chooses. This Quirk is equally potent on offense and defense, and that's true even when Dark Shadow is at its weakest. At night, this Quirk grows even bigger and stronger, outclassing what even most pros are capable of. Combine that with Fumikge's smart and cool-headed disposition, and his lack of difficulty in mastering this Quirk, and it's tempting to say that Fumikage is downright OP. He hit the ground running with this Quirk, like Shoto and Bakugo, but My Hero Academia smartly nerfed him in a few key ways. Otherwise, Fumikage would have been rather stale.
Fumikage's Humble Quest For Growth & Development
Fumikage is balanced with a few notable drawbacks and some distinctive room for improvement. At the series' start, he easily had Izuku beat overall, especially since Izuku struggled to safely use One For All's overwhelming power. Fumikage's Quirk seemed perfect at first, but the cycle of day and night slows him down in two ways. During the day, his Quirk can't use any finishing moves or a sudden burst of strength, being limited by sunlight. It's strong, but it can't pull off something like a 1,000,000% Smash in a pinch the way One For All can. Also, sudden bursts of light, such as from Katsuki Bakugo's explosions, will weaken it.
If Fumikage goes pro and his weaknesses become clear, villains can start using Quirks or equipment to weaken Dark Shadow with strong light. And during nighttime, though Dark Shadow grows stronger, it becomes that much more difficult to control. During the Forest Training arc, for instance, Fumikage totally lost control of Dark Shadow and went on a terrifying rampage, attacking friend and foe alike in a blind rage. That's... not good.
Fumikage clearly has room to improve. His true potential is a double-edged sword and he is training hard to overcome that problem -- just like Izuku did by training with Gran Torino to control One For All's incredible power without it going wild. In that sense, Fumikage and Izuku are in the same boat, even if Fumikage's Quirk seemed perfect at first.
What is more, Fumikage has a fairly frail constitution, being a short and slender boy with little muscle. If someone slips past Dark Shadow's defenses or takes Fumikage by surprise, then he is practically helpless. Later in the series, Fumikage trains with Hawks the Pro Hero to put on some muscle, showing that in more than one way, Fumikage had substantial room for improvement. And Izuku would also come to know a thing or two about putting on muscle to become a true Pro Hero.
The final thing holding Fumikage back is that he lacks clear motivation to become a hero. It's a given that every student in U.A. wants to be a pro so they can save lives and perhaps get some glory, but what else drives Fumikage? Not much, it seems, and this lack of personal drive may be holding him back. Izuku and Bakugo each want to replace All Might (their idol), Ochaco wants to live up to Izuku's example, Tenya Iida wants to honor his big brother Tensei, and even Minoru Mineta has a clear goal, to impress all the ladies as a Pro Hero. Fumikage is lagging behind, and if doesn't figure out his dream soon, his heroic quest might stall out, no matter how strong his Quirk. At the very least, this gives him more room to grow as a character: the best might be yet to come.