When Aerith Gainsborough dies, put X +1/+1 counters on each legendary creature you control, where X is the number of +1/+1 counters on Aerith Gainsborough.
Aerith grows herself by gaining life every turn, then turns into a payoff when she dies by dumping all her +1/+1 counters across your other legendary creatures. You build a mono-white go-wide-on-legends or lifegain board, protect or sacrifice Aerith on your terms, and win through oversized lifelinking attackers or a legendary swarm.
Lifelink plus the gain-life trigger makes Aerith a self-growing threat with built-in life buffer
Mono-white keeps the manabase clean and gives access to the best lifegain and counter-doubling enablers
Death trigger rewards a legendary-heavy board, turning one removal spell into a board-wide buff
Lifegain synergies are deep in white, so the engine is easy to assemble
Effectively a one-shot payoff: once Aerith dies and distributes counters she's gone
Slow and grindy with little interaction or card advantage native to mono-white
Heavily reliant on having other legendary creatures out to make the death trigger worthwhile
Vulnerable to exile, -X/-X, and counter removal that sidesteps the death trigger
Lifegain alone doesn't close games without a payoff