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Dream's HP is constantly fluctuating!!(I know this isn't how HP or HOPE is portrayed in the game if I'm remembering correctly. This idea stems from the idea that Sans has such low HP due to his bad mental health).
This comes from the influence of the Positive Apple inside of Dream's soul! That apple's existence defines Positive things- peoples dreams, their wishes, past and current achievements. The thing every human, monster, and animal feels when they see their loved ones, or spot a crying newborn baby with a button nose. But the feeling isn't consistent for everyone, nor the entire universe. These things the apple exist on directly impact Dream's HP- his HOPE.
His range of HP is influenced by how Dream is currently feeling. Say his average HP when hopeful is 550-650, his range of HP when he's losing hope is from 500-600. It's not stagnant, and has dipped lower than that on heavy, dark days for Dream. But every single fight his HOPE is a different number since the positivity levels and Dream's mental health never stay the same. It's uncommon to not see Dream's HP changing day by day, and often Dream feels conflicted when its stable because that's just not normal. Sometimes, after a series of attacks of AUs which are severely impacted (he failed to protect them), Dream has experienced his HP dipping below 300 as a result.
(Truthfully, Dream doesn't know if he can actually die or not. He isn't going to test it, but on those multitudes of hopeless days of his foot stills right before the golden portal.)
Although due to Dream's childhood trauma, clinging onto his brother's last words, and trying to manage his way in completely new worlds while fighting said brother- his own HOPE has almost never reached its max amount.
I imagine in AUs where multiversal balanced is considered when Dream first breaks free from Stone he's easily below 50 HP. Then until balance is resorted, he'll never reach a "healthy" amount of HP in comparison to the gods, their followers, and the negative au's he's regularly fighting (or his HP as seen in a balanced multiverse). It's not uncommon at all for Dream to be the one with the lowest HP out on the battlefield until Blue comes along.
Then when HOPE is abundant, Dream becomes a literal tank. It's more likely Dream's going to run out of stamina before being taken down. Yes, he takes hits for his allies in these situations! It's often a hopeful spiral up from here. But even with the HP presented prior in a balanced multiverse, Dream's still someone that can take a lot more hits than your average combatants!