Nurse

Three things have to be true at once: a vacant house, the Merchant already living in town, and a character whose maximum life is above the starting value, which in practice means at least one Life Crystal used. That last condition is the one people miss when they wonder why she has not shown up in a brand new world.

Her heal does not touch the potion cooldown. That is the whole reason she is written into boss strategies rather than treated as a convenience. Set your spawn point beside her house, keep a Magic Mirror on the hotbar, and a moment that would have ended the attempt becomes a trip home, a full refill, a cleared debuff list, and a trip back. In single player the pause key gives you as long as you need to line the sequence up.

She does not charge a flat rate. Each major boss you clear raises what she asks, her happiness moves it up or down by a wide margin on top of that, and expert and master worlds double the result outright. By late game a full heal costs real money, so a strategy built on repeated mid-fight healing needs an income behind it. This is one of the few cases where keeping an NPC happy pays for itself directly, and a Discount Card now applies to the healing as well as to shops.

She bills per debuff on top of the life restored, so walking in poisoned, burning and cursed costs more than walking in half dead and clean. If you only need the debuffs gone, the heal option still appears at full health.

She strips everything at once, which is usually what you want and occasionally not. Drink your Ale after the visit rather than before, since Tipsy counts as a debuff and she will clear it along with the rest. A short list sits outside her reach and she does not bill you for those either. Potion sickness is on it, so she cannot be used to skip your healing item cooldown, and neither is standing near a Water Candle something she can undo.

Left near enemies she throws syringes. The green ones damage and poison, the red ones heal her and any NPC standing close, and the healing throws home in if the first attempt misses. She keeps herself alive in situations that would kill any other townsfolk, including underwater indefinitely. The catch is that she cannot heal herself while you have her dialogue open, which matters if you are topping up during a Blood Moon with something chewing on her.

Build her a house near the arena rather than dragging the whole town to the fight. She only needs to be at your spawn point for the mirror trick to work. Check her happiness before the fight instead of after, because the gap between a content nurse and a miserable one is large enough to change whether the strategy is affordable.

• The Arms Dealer is written as being interested in her, and other townsfolk talk about them as a couple. She and the Mechanic are written as arguing during a Blood Moon.
• A painting called Nurse Lisa exists, which is the Mona Lisa with the obvious substitution.
• One of her lines complains about the lack of social distancing in a crowded town, added when Journey's End launched.
• Her comment about how often you have died is a nod to banners, which most enemies hand over every fiftieth kill.
• Shimmer gives her an alternate look. It changes nothing about her behavior or dialogue and can be undone the same way it was applied.