Rance IV:Secrets

Rare Items
Some non-essential items are rare drops inside Treasure Lumps around certain general areas. There's no tricks for getting them except grinding. The rewards are very good, however.
 * Research Core BF5, Belief Key. This can be used to access the locked western room in Surface Central Area for the Buster Sword.
 * Defense Core multi floor, Betrayal Key: Hanny Knife
 * Buoyancy Staff F4 (also obtainable on 1F but may require being past Chapter 4), Chernen Powder: The item shop becomes Rance's. Everything costs no gold, and Yocchan becomes Rance's as well (new CG).
 * Tousho Core
 * Epson key : Locked room on east side of Tousho Core B3F, has a chest with the Angel Robe.
 * Sharp key : Locked room on east side of Tousho Core B3F, has a chest with the Coarse Armor 3.
 * Fujitsu key : Locked room on east side of Tousho Core B3F, has a chest with the T34D Trial Shield.
 * NEC key : Locked room on east side of Tousho Core B3F, has a chest with 3000g.

Mirror Fragments
Throughout the game, you can find Mirror Fragments. Bringing 4 fragments from the same mirror together saves the girl from the mirror, and you get a reward.
 * Set A, Rukira: 2000 gold
 * Set B, Ella: Lightweight Bow
 * Set C, Gezona: Lionet
 * Set D, Oichi: Strength Spray

150 Summons
If you summon Willis a staggering 150 times (summoning Johnny while in control of Maria does not count) she will offer you a prize of your choice:
 * 1000 GOLD.
 * A strip show (a unique CG featuring her and Mikan).
 * "God Armor (c)," the most powerful armor in the game at 45 DEF.

Mikan's Mechanics
For anyone who wants to spoil the surprise, Mikan's mechanics are as follows:
 * She will show up on a random chance of 1-in-5 if Rance has at least 3000 EXP (there is no way to "cheat" her by losing only a small amount of EXP).
 * Mikan's first few attempts are actually pre-scripted:
 * 1) She will screw up and set Rance's EXP to 0.
 * 2) She will enhance his weapon.
 * 3) She will screw up and set Rance's EXP to 0 again.
 * 4) Rance's EXP will triple.
 * All further attempts are genuinely random:
 * An 84% chance that she will 0 your EXP.
 * An 8% chance she will upgrade your weapon.
 * An 8% chance that she will triple your EXP.
 * In short, it is only worth using Mikan's services the first four times, if at all, minimizing your losses and maximizing your gains by save scumming. And yes: she will never get the ceremony correct the "normal" way.

Rare Monsters
There is a 1 in 100 chance that any Green Hanny you encounter is actually a Happy Green Hanny, worth a base 1000 EXP instead of 30. They can only be distinguished by their name and payout.

There is a 1 in 64 chance (and only in certain enemy formations) that Treasure Lumps will become yellow treasure lumps worth 501-1000 GOLD.

Shivan
The hidden monster Shivan (seemingly named after a member of Alicesoft's sales team) can be found in South Tower if you discard at least 25 items over the course of your playthrough. Once you're done discarding, go to the ground level of the South Tower (the one in the middle of town) and head down the leftmost hallway. In the middle of the hallway in an otherwise inoccuous space, Shivan will fight you.

Shivan is immune to most damage types and can only be hurt by Ice and Dark attacks, so you may want to come back later in the game. Despite all the fuss involved in finding and killing him, Shivan's only reward is EXP (albeit upwards of 6000 depending on your level).