Jack is wearing the same torn shorts in that shot as he was in the final scenes of "XCVI," and the promo shot above stands out from the others in similar fashion. Here Jack is at peace, the green coloring evokes nature and serenity, not the eerie X-Files-level creepiness of the Green Samurai. It's worth noting that the scene where Jack lost his sword was also clouded in green. I think it's clear that given the promo image, Jack's fear paired with a familiarly furrowed squint of resolution, and the fact that he spent an entire episode -- thanks to Ashi -- re-upping on how evil Aku's chokehold could be, the Green Samurai has to be some ideal version of Jack -- or at minimum an ally that helps him become that person.
It's not Death because Jack wouldn't resolutely walk to his death, not after reminding himself how precious life was; moreover, Death traditionally rides a pale, sickly horse, and the Green Samurai's is upright, full-bodied, and stately. It's not his samurai ancestors -- though the armor looks like Jack's father's armor from early seasons -- because we've established that they appear in fire and brimstone and it'd be a much weaker thematic reveal. It's not the sword itself because Jack is separated from it; if it were simple enough to send him this vision, he'd probably have gotten it back by now.
The Green Samurai is something more. Visually, the green pastures of that promo image contrast with the reds and blacks of Aku's mechanized city that we saw again in this episode. which have routinely been the visual identity of villainy in Samurai Jack. This season renewed the show's emphasis on the sanctity of human life, which positions Jack as a guardian of that life. The Green Samurai's antlered helmet echoes Jack's own armor design from early in the season. Thematically, it makes no sense that he'd be revealed to be anyone other than Jack himself (likely a 2017 update on the bearded version of him that appeared in the Season 3 episode "Jack and the Traveling Creatures"). I'd argue that revealing him to be anyone else would feel hollow.
One more thing: The design also echoes that of historic samurai Honda Tadakatsu, who one site has described as "a face-obliterating samurai ass-kicker" who favored a spear and was called "The Warrior Who Surpasses Death Itself" in his day, much like Jack does. Whether or not those details matter (they do), this green warrior is clearly an ally to Jack, and will help him recover his sword, probably in the next two episodes.
"It is time," he says.
Jack agrees, and we do too.
While we wait, check out the season 5 episode 6 preview below. We'll get to see the Woolies again!