Goku (False ssj) JUS

When he transformed into the true Super Saiyan during the fight against Frieza, Goku witnessed the murder of his best friend, Krillin (as well as the fact that Krillin is not likely to be revived with the Dragon Balls, as Shenron had already granted that wish and could not resurrect the same person twice), something that drove him over the edge. This ultimate pain was not fully present during the fight against Slug, that and Goku wasn’t at the same power level as when he fought Frieza, which is most likely why Goku did not transform fully. The form of False Super Saiyan was shown to be very erratic, lasting for only as long as the aura is ignited, and using up much more energy to sustain than the full Super Saiyan.