The Ultimate Dimensional Robo, Great Daiyusha has served me well since August 2013, when I started Vanguard. My very first deck attempt mixing Platinum Ace and BT-08 Dimensional Robos together. Then as time passed I went full Robo, with reinforcements sent along Dimensional Robo, Kaizard to combat Link Joker, a force of Invaders possessing the power of LOCK, lead by Star-vader, Chaos Breaker Dragon, whom alongside the multiple other generals of the Star-Vaders also wielded the power of Reverse.
During the invasion of Star Gate, the Dimension Police and Nova Grappler fought fiercely to protect their territory, however they sustained heavy losses with powerful warriors from both sides becoming corrupted by "Reverse"
Now this next section is based upon my own hypothesis of what happened after Daiyusha was Reversed. The lore never explained the outcome of what happened next.
Super Dimensional Robo, Daikaiser was forced to fight the hero who had now become Dark Dimensional Robo, "Reverse" Daiyusha. The confrontation would prove to be fatal, the fallen hero was saved by Daikiaser, however I( expect) the damage endured was to great, Dark Dimensional Robo, "Reverse" Daiyusha, was destroyed, but his core, now liberated survived.
In the aftermath of several battles, Star Gate lay in ruins. Super Dimensional Robo, Daikaiser remained on the ground along with a small number of companions as they raced to the Front Lines of the war alongside Nova Grapppler Beast Deity, Ethics Buster to join up with the Gold Paladin Liberators, Shadow Paladin Revengers, the Aqua Force troops under the command of Blue Wave Dragon, Tetra-Drive Dragon, and others.
However, with damage Kaiser Grader would have sustained int he battle for Star Gate, my guess would be that this is where Super Dimensional Robo, Shadow Kaiser steps in. or rather specifically the Kaiser Wing.
The Link Joker advance had reached the United Sanctuary and it was there that Star-vader, Chaos Breaker Dragon unleashed his ultimate weapon, a being made from pure negative energy harvested from those he'd "Reversed"
The Doomsday weapon, designation: Omega, was defeated by the Liberator Army while the armies of other regions of Cray fought with the various generals of the Star-vader forces. With his weapon destroyed, Star-vader, Chaos Breaker Dragon retreated, leaving his army to fend for itself, with the loss of their commander the remaining Generals were defeated and shut down.
In the aftermath of the Reverse War conflict came a time of reconciliation and rehabilitation, Dimensional Robo, Daiheart's Core was probably transferred to a new body, that new form being Super Dimensional Robo, Diard, it's power equating to being an upgrade of Super Dimensional Robo, Daiyusha, however the trade-off being that the reborn Daiheart losses the power to combine with he Rescue Yusha, however we can expect that these would have been destroyed along with Dark Dimensional Robo, "Reverse" Daiyusha.
I can work out a logical explanation to the origins of Diard, however what if Daiheart's Core was instead placed into the Kaiser Wing allowing Shadowkaiser to act independently of Dimensional Robo Kaizard? This is where the time-line also starts to become skewed making any plausibility even harder to calculate as the result is an endless stream of alternating timelines and possibilities, including timelines in which Daikaiser and Daiyusha never fought off against each other, and the latter never even being "Reversed".