Kanda has sword skills that make him so much stronger than her that it’s spooky. Lenalee just can’t take on the power of Kanda. Kanda is too powerful and has thunder so he cannot be defeated. Kanda is a swordsman of a great level. Kanda wins.
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I don’t know what powers he shows later in the series (don’t tell me, I’d like it not to be spoilered) but Lenalee seemed really strong last I saw, while Kanda just fired those underworld beasts from his sword. Mind you the latest opening has shown Kanda with two huge glowing swords
Don’t worry, I won’t spoil it. It’s definitely annoying when someone spoils something for you. Well, later on his attacks are mostly the same. He’s just better now (Thanks to time skip) His sword may be a little different and he probably has more attacks. I lost track of the attacks in the manga since they don’t have easy names to remember. Lenalee’s tough, but I found Kanda to be more of a fighter