Synthesis is a new mechanic that will be unlocked in item creation as soon as Sarah joins the team. In simpler terms, synthesizing an item means that you're upgrading it. The good thing about synthesis is that you can combine even the simplest food item to a weapon, armor and accessory. Be careful though; synthesis is permanent. An item can only have up to four factors or additional effects.

Synthesis is fairly simple. Choose a weapon, armor or accessory you want to synthesize, then choose an item you want to combine it to. The game will let you see a preview of the bonuses to help you finally decide what to combine. Different items have different effects; some add ample bonuses to the stats or provide additional effects like auto-counter or attack absorption.

Some items that you may consider combining are the following:

Natto, Barrier Breakwater - +10-15% resistance of all status ailments
Crystal - increase chances of critical hits
Angel Feather + - +20 to all stats
Seeds - +XX% to various stats
Various foods - +FOL, +EXP, various stats
Fainting Potion - +80 guard
Minerals - add elemental damage to attacks, shoots elemental projectile when attacked

Synthesizing an item will change its name (e.g. "Item-R01") to help you identify which item you have synthesized, if ever you have multiple copies of that item. Accessories are your best option to synthesize, since you don't need to change them often unlike weapons and armors.