Understanding the physical characteristics of small Solar System bodies is important not only for refining formation and evolution models but also for space mission operations. Although several kilometre-sized asteroids have been visited by spacecraft, asteroid 1998 KY26—the final target of Hayabusa2#, the extended mission of the Japan Aerospace Exploration Agency's Hayabusa2 spacecraft—will be the first decametre-scale asteroid to be explored in situ. Its small size and rapid spin place it above the upper limit on the rotation rate, indicating it may differ from previously studied bodies. In this work, we conducted a photometric campaign during 1998 KY26's close approach to Earth in 2024, revealing a high optical albedo and Xe-type colours. We determine its spin period to be (5.3516 ± 0.0001) minutes—half the period of earlier estimates. Lightcurve inversion produces retrograde pole solutions in both convex and non-convex shape models. Combined with 1998 Goldstone radar data, these results give a diameter of (11 ± 2) m, three times smaller than previously derived values. The derived cohesive strength levels necessary to keep the structure intact, which is less than 20 Pa, suggest a possibility of the asteroid's rubble pile structure, though this finding does not rule out its monolithic structure. These results can be validated with future James Webb Space Telescope observations. Our comprehensive characterisation can inform the planning of the Hayabusa2# rendezvous in 2031 and helps pave the way for future studies of dark comets.