Mimic

The Mimic is a monster mimicing a treasure chest found in the Treasure Cave. She resembles a man-eating chest, lined with teeth when opened and revealing what seems to be a pale, undead girl with black hair and clothes. When closed she looks exactly like a normal chest.

Luka has the option whether to ignore fighting the Mimic. There is no reward, unlike other monsters, for skipping the fight with Mimic. However, Luka will gain experience if she is defeated, making the upcoming battle with Nanabi a little easier. Alice does chastise Luka for opening the chest in the first place though.

Encyclopedia Entry
''“A monster that hides in a treasure chest, it appears to be a part of the undead family. Their body is composed of the chest and their digestive juices contained inside. When an adventurer opens the chest, the Mimic will drag them inside and close the lid, bathing them in digestive juices.''

''A very timid monster, it spends almost it's entire life shut inside the box but once she catches her prey, her attitude completely changes as she enjoys dominating them. Dragging her prey inside of her, she enjoys watching their despair as they are slowly eaten. Moreover, Mimics will be sure to cover the male's genitals with her digestive juices first, cruelly watching their reactions. A man captured by a Mimic can only look forward to a disgraceful, miserable death.''

''There have been cases where the Mimic really enjoys the reactions of her catch and chooses to instead torment them for the rest of their life, or until they become bored. When this occurs, they will then eat their prey.”''

Attacks
Withdrawal: Closes Mimic's box, any attacks in this stance lead to a Swallow counterattack.

Slow Swallow: Triggers bind status and causes damage. After the fourth consecutive attack she will draw Luka into her box. Will trigger facial bukkake on losing.

Swallow: Counterattack which leads to Digestion on the same turn.

Digestion: Follows Swallow and delivers a one hit KO via instant follow-up.

Battle Overview
Despite being optional, Mimic is a very difficult battle, especially at the stage fought in the game. She will spend a turn either being open or closed and the objective is to hit her while she is open. If Luka attacks her when she is closed, she promptly uses Swallow as a counterattack and end the battle, using Digestion relentlessly to finish Luka off.

Even when she is open, she isn't a cakewalk, after revealing herself she will use Slow Swallow, in which case requires many turns of Struggling to break free resume attacking afterwards. Luckily she will sometimes just stand there, allowing Luka to land a hit without being attacked in return.

Depending on how Luka lose, there are two scenes. Losing to her Swallow/Digestion counterattack will have Luka being dissolved and eaten by the Mimic. Losing to her in any other fashion will have her dragging Luka into her box where she toys with him for all eternity.

Trivia

 * After being drawn in by Slow Swallow, Mimic will use an unnamed attack constantly until his HP runs to 0.
 * Mimic is obviously a homage to the various Mimic based enemies in various RPGs where treasure chests reveal themselves to be powerful monsters.