I am considering staying at Hotel Asakusa & Capsule for a few days.
I know I have to leave the hotel during the day for cleaning, I don't mind this as I will be going around town anyways.
However, I would much rather not have to carry my main backpack everywhere I go and only carry a small bag for daily needs.
Is it possible to leave my belongings in the including locker since I would be staying there for 3-4 days in a row, or do I have to carry all my belongings every day?
As a rule, capsule hotels are not set up for multi-day stays. However, this one appears to be an exception, and you can keep your locker during the day - with the major caveat of not being able to enter the hotel (and access your belongings) between 9:30AM and 4PM. Beware that capsule hotel lockers may be quite small and a full-size backpack may not fit.
That said, while capsule hotels are a fun "only in Japan" novelty and worth trying once, I would not recommend multiple nights in one. Your privacy and space is quite limited, they're noisy with drunk people coming and going all night long, and not being able to use the room during the day can be quite a disadvantage when you're jet lagged. You can likely get a "normal" hotel for a similar price if you book in advance: I've stayed in the same chain's Tsukuba Hotel in Ueno, and it was fine and the Japanese-style room (with shared bathroom) only costs ¥3800/night.
