The game mechanics description in the OP is wrong in some areas. Take or leave the content below, I'm leaving it for my personal reference:
HP: The round is lost if it hits zero. Player ends up back at the control room believing everything is a dream, but keeps all of her items, levels, perk points and skills learnt. Certain perks once learnt will give the player a % chance of HP regeneration after each exploration turn. Certain items acquired from enemy loot will restore a % of the player's total HP pool.
SP: Used to perform skills acquired from perks. Like the HP mechanic, there is also a skill granting a % chance of SP regeneration after each exploration turn. There are a lot more items which restore SP than HP in the early game, so players are encouraged to actively use their skills from perks when the situation calls for it.
VP: The game calls it the "virus meter". If it hits 100, it's the same as losing a round from 0 HP. The VP goes up each time a zombie ejaculates into the player during rape scenes, each ejaculation is 40 points but this can be reduced by spending the perk points on the appropriate skills which unlock as the player is subject to an increasing number of vaginal ejaculations. Outside of combat, the only way to reduce VP is to consume the antiviral pills (抗生物質)which will decrease the VP count by 20 for 3 exploration turns. VP is also reset to zero when players successfully complete a round by defeating the boss at the end of the zone.
Adrenaline: The meter on the right-hand side with the heart symbol at its base. This increases during sexual acts, through consumption of the doping consumable and through random events (gas). Besides changing the expression of the player during rape scenes, it also grants a % bonus to damage, so players are encouraged to balance damage buffs with available antiviral pills to prevent rapes from losing the round. Like vaginal ejaculations, climaxing during rapes enough times unlocks certain skills (for example guaranteed escape from attempted rape by spending SP, reduced active gauge penalty during penetration, HP, SP recovery during sex acts etc.) which can be activated using perk points. Adrenaline naturally decreases to zero if turns pass without sexual acts or drug-induced increases, so players aiming for climaxes will need to balance rapes with available antiviral pills.
As of demo v0.2, the climax-related sex skills have up to 6. Number 7 is unnamed, so that will have to wait for a future version. Sex-related skills only apply when she's being raped, and possibly afterwards when she's down for the count before getting up. Since the enemies do not take the chance to hump her when she's down, the skills relating to HP and SP recovery during sex mean free recovery at no cost to items.
Inventory: There is a hard-cap for the different categories of items available. For example, bread is limited to 3 loaves, lollipop 3 sticks etc. Some of these limits can be raised by investing in the appropriate skills after their blueprints have been found in the zone. Any excess items found will be automatically transported to storage, which is infinite. When the player returns to the control area, the storage will automatically top up the item quantities to maximum from available stocks.
Active-time gauge: Both the player and enemy faces will move along the gauge from left to right, with the rate of increase dependent on each party's AGI stat. At the 75% mark, the gauge is marked a different colour. When the player hits 75%, a basic action like kick or shoot can be used which resets the gauge to zero. If the player elects to breathe (the shield icon), the player's icon will return to the 50% mark after hitting 100%. Certain special moves (e.g. throw grenade) will only activate once the gauge hits 100% and CAN be cancelled if the enemy executes a rape grapple before this happens.
Enemies who have an active gauge of between 75-100% can be COUNTERED by the player using the "Kick" command. When a kick is successfully executed on an enemy satisfying the above condition, the enemy loses the intended action and the active gauge is reset to zero. This is why the game advises players to use the breathe command, as enemies with slightly higher AGI than the player are likely to hit 75% first followed by the player, leaving them open to a kick counter.
Rape-capable enemies who have telegraphed their rape-grapple move will instantly execute it when their gauge is at 75%, NOT 100%. In exchange, ANY offensive action which depletes their HP will cancel the attempted rape, but not reset their active gauge value.
Zone guide:
Underground containment facility (地下保障施設) (sic)
Rape-capable enemies: Male zombies, zombie dogs, cyborg boss.
Zone gimmicks:
Sometimes a player will be given three choices when the path branches, available choices ranging from regular enemy, chest, mysterious merchant and boss enemy. The third option is random (marked by the question mark)
A cat will sometimes appear asking for fish. If players have it, they can feed the cat the fish who will then run away. Several turns later, the cat will return with an item to thank the player for her generosity. This consumable item is a permanent stat booster (HP, SP, AGI), so keep fish on hand for this zone. Fish are available to buy from the mysterious merchant after clearing the zone the first time.
The cyborg is always accompanied by a zombie and zombie dog. Throwing a grenade is usually enough to get rid of the flunkies, leaving the boss as a 1-on-1 fight. The boss is rape capable, so it's easy to stun it when it's preparing to rape. It does have a punishing combo of blows as a normal attack, so use flashbang grenades to keep it immobilised if HP is dangerously low. Just about anything with high damage will finish it off. I personally like coming into this fight with high adrenaline for the damage boost to the opening grenade throw, then letting the cyborg rape and climax the player for the purpose of skills development.
Greenhouse area (温室エエリア)
Rape-capable enemies: Mutated plants, anacondas
Zone gimmicks:
Chests can be trapped, whereupon the anaconda inside will ambush the player for an immediate attempt at rape.
If the player showed the robot at the start of the zone the scientist's ID card, it will give the player a molotov cocktail at the end of the scene. Later on (around 2/3 to 3/4 of the way through the zone), the robot will offer to provide a "service" to the player. Accepting it will increase adrenaline with a choice of either VP reduction, doping or a small HP + SP recovery. Accepting the "service" counts towards vaginal ejaculation for the purposes of perk skills.
Plant-based enemies are vulnerable to fire, making molotov cocktails seriously useful.
Bring water along when taking on the zone boss. Because its AGI is so high, flashbang grenades help reduce HP loss from its annoying attacks.
Impregnation mechanic:
Occasionally the post-orgasm scene will have a special effect, where the enemy's seed has impregnated the player. The type of enemy involved has different modifiers, but all seeds have one positive and one negative modifier. Players can get multiple copies of an enemy seed, and there is one skill in particular which increases the probability of impregnation. List of effects given below:
Zombie: ATK +5, AGI -1
Zombie Dog: AGI +1, Max HP -50
Fat Zombie, Max HP +50, AGI -1
Cyborg (1st stage boss), Max SP +10, AGI -2
Chopper (Venus flytrap), Max SP +5, Damage taken +1%
Ivy Zombie: ATK +5, Max SP -5
Anaconda, ATK +5, Damage taken +1%
White variant zombie, Max HP +1%, Max SP -1%
Failed experiment (the sludge in stage 3), Damage taken -1%, AGI -1
Nurse, Max HP +50, Critical rate -1%
Administrator (the flying drones of stage 4), Critical rate +1%, Max HP -50
The stage 2 boss has a sex attack, but his seed is the same as the zombie's.
Use the Eve potion to restore virginity and clear the seed effects.
As of 0.9, recommend the Zombie, Zombie Dog, Ivy Zombie, Failed Experiment and Flying Drone. Max HP is not really a problem when there's a cheat item which fully restores HP even in battle, while AGI debuffs can be countered by lots of zombie dog couplings. Max SP caps are less of an issue, because each level-up raises max SP and there'll come a point when players will have more than enough when items are taken into consideration. Increased critical rate and ATK speeds up fights, so those are top priority even with the attendant debuffs. Anaconda isn't recommended because taking increased damage together with debuffs to max HP is too much of a risk.