wilson's heart is less about amber, but about how others, especially house and thirteen, saw amber. the episode opens, post credits, on each of the characters watching amber through glass. if this were a normal potw, we would've met their friends and family, like foreman's father in euphoria. if this were any other patient, we would've learned what was motivating their drug use when the team finds diet pills and amphetamines, but here, the revelation serves only to show how the characters (including wilson) didn't know her as well as they thought they did. we see only a fleeting glimpse of the real amber.
house's visions of amber are stylized, flickering between potential sex object and purveyor of wisdom. she is even costumed differently, in sleek pantsuits, where the real amber wore prim sweater and skirt sets. in the wake of several episodes of the show telling us that amber is a female house, house's amber is an embodiment of his subconscious: his imaginings of what could have been, his guide to what just evades his memory, and the voice of what he knows is true.
for thirteen, amber is like a shadow self. they were positioned as rivals in the games arc and their fates are inextricably linked: because of thirteen's mistake in 97 seconds (incidentally the last episode where the patient dies), foreman was brought on and house had to make his gender play to hire three fellows--meaning he had to pick between thirteen and amber. house picks thirteen, and wilson was drawn to amber in her hour of need. if any of these dominos had fallen differently, maybe amber would've been working that night, along with the rest of the team. maybe it would've even been thirteen who was seeing wilson and who got house's call that night--after all, when house learns about wilson and amber, the first thing he asks is, "she's not dying, is she?"
though house dismisses it offhand, i think thirteen is stricken by guilt. like house's role in amber being on that bus, thirteen played a role in amber ending up in this position, however attenuated. now, having won the fellowship over amber on, essentially, a coin flip, thirteen is watching amber seemingly dying in her place. thirteen couldn't even touch amber because she bear to be another link in the causal chain causing her death--because it should've been her.
but there's more than that. if amber resented thirteen for the apparent ease with which she moved through life, i think thirteen saw in amber a kind of fearlessness she lacked. where thirteen was happy to fade into the background until house picked her out, amber took her own fate into her hands. where thirteen--living with damocles' sword over her--has always had one foot out the door, amber extracted every last drop from life, whether that was a fellowship spot or a deal on a mattress.
amber's death shook thirteen so much because to thirteen, amber's mortality felt impossible. wilson's heart is so emotionally effective because it plays on the injustice of the situation. house is supposed to do the impossible, but when it mattered most, he was powerless. where house's superpower is diagnostics, amber's was playing the system. she was supposed to have found a way to save herself even if she had to screw over everyone else. in thirteen's mind, amber was supposed to bury her, not the other way around. but by trying to give herself a minuscule advantage over the flu, amber sealed her fate.
it is as if thirteen and amber traded places. for the hour or so she is awake, amber is in the same position as thirteen post-diagnosis: alive, with full, graphic knowledge of her inevitable death. cuddy thinks that amber would've wanted that last hour anyway. that's probably true but amber doesn't get to choose. amber makes only one choice: she tells wilson that it's time. amber--amber who fought everything tooth and nail--goes to her death without a flicker of rage.
thirteen, paralyzed by indecision, makes two choices: to see amber and to be tested. in one of the final episodes of the series, wilson asks thirteen what it's like to be dying, and maybe thirteen, seeing amber on the precipice of her own diagnosis, wondered the same thing. if house was reluctant to rejoin the land of the living, thirteen has been on the bus her whole life, in the liminal space between life and a truth that was too hard to bear. thirteen thought that not knowing made her braver, but being brave and bold and brilliant didn't save amber. so for once in her life, she does what amber would've done, if she had the choice: she gets off the bus.