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If he was smart enough to realize Leon was gonna lose to the MC then the rush could make some sense. But they don't elaborate on Leon and Rose's relationship beyond the fact they professionally know each other. If Rose was Leon's primary sponsor, then Rose could have financial gain by turning Leon into the Galar region's savior, not just the Champion, as his primary motive. Instead Rose seems intent on becoming the Galar region's hero himself since he can't measure up to Leon as Champion but he's reliant on Leon to save the day so it doesn't really make sense. If there's no financial gain to get from Leon becoming even more famous worldwide, then I don't really see a reason why he'd need Leon to be Champion at all. He just needs any strong trainer that loves the Galar region to catch Eternatus. No matter which way you look at it, there's too much necessary information missing.