Strengthening Synthesis / 強化合成, commonly shortened to just "synthesis", is the process of giving Materials / 素材 to a flower knight in order to power them up. The flower knight being powered up is called the Base / ベース. Materials can include Manyus, Ampules, Skill Flowers, Equipment Flowers, and even other flower knights. Strengthening Synthesis can be done by going to the Character Management page by clicking on the Character / キャラ button and clicking on the third tab labeled Strengthening Synthesis / 強化合成.
The effects of strengthening synthesis include leveling up your flower knight, increasing their skill levels, and unlocking their equipment slots. You can feed up to 10 materials to a flower knight at once. It costs gold to do a strengthening synthesis. A synthesis can sometimes result in a Great Success / 大成功 which further boosts the experience gain you get from materials.
All materials have an attribute associated with them: slash, thrust, blunt, and magic. If you match the type of the material to the flower knight being used as the base of the synthesis, the material will give 1.5x more experience. Although Ampules have attributes as well, they give the 1.5x experience bonus even if their attribute does not match the knight that is receiving them.
Strengthening Spirits: Manyus / 強化霊 マニュ, or Fairies, are the most common thing you will be using as synthesis materials. The come in three rarities which offer more experience based on their rarity. Of all of the materials you can synthesize a flower knight with, these offer the most experience.
Equipment flowers / 装花 and skill flowers / 技花 are character synthesis materials you get from events. These are character-specific materials that can be synthesized with an event flower knight. The two types of flowers guarantee unlocking an equipment slot or increasing her skill level respectively. By contrast, synthesizing a flower knight with herself has a chance to boost her skill's level. The flowers have names signifying that they belong to a particular flower knight. For example, Pine Tree's Equipment Flower / マツの装花 being only usable by Pine Tree / マツ. For all characters, the skill flowers are outlined in pink, and the equipment flowers are outlined in blue.
Kodaibanas / コダイバナ, or Ancient Flowers can be synthesized to any flower knights with matching rarity or lower. Kodaibana skill flowers can be bought from Wares with LC and sun medals. They guarantee skill level ups of a base flower knight of the same rarity or lower. Kodaibana equipment flowers are premium items that may be bundled in Gachas (at minimum, 3k DMM points). They unlock an equipment slot of a base flower knight of the same rarity or lower.
Other flower knights can be used as materials for synthesis. If the base flower knight and flower knight used as a material are exactly the same person, then the base flower knight is guaranteed to have an equipment slot unlocked for each material that is the same flower knight. Furthermore, if the base flower knight and material flower knight(s) have the same skill, then you have a chance to increase the base flower knight's skill level. See #List of Flower Knights With Common Skills
Sacred Beast: Ampules / 霊獣 アンプルゥ permanently increases a stat for a flower knight. Up to 100 of each type of Ampule can be given to a flower knight. Ampule bonuses are retained throughout all evolution stages. Note that Ampules may have other similar names because their name is spelled very strangely (Japanese players also get it wrong).
After 100 Ampules of a given type has been given to a knight, that knight can be given up to 100 High Ampules to further increase their stats.
Awakening Dragon: Centy / 覚醒竜 センティ increases a knight's max level by five. This gives a stat boost equivalent to 25 Ampules in each parameter. Centies can only be used on Bloomed 6*s, so Rarity Growth is required for lower rarity knights. They can be bought for 25 Rainbow Medals or refined from 100 Petite Centy.
1. Go to the strengthening synthesis / 強化合成 tab, select the base character you want to power up, then press the red confirm / 決定 button at the bottom-right.
2. Click on any of these dark boxes to select materials / 素材.
3. Select materials stored in your warehouse / 倉庫 to give to the base character.
4. Select the number of materials to use. At the bottom, press the red yes / はい button on the left to use the materials, or press the cyan no / いいえ on the right to cancel.
5. Repeat step 3 until you have all of your materials selected. Press the red confirm / 決定 button at the bottom-right when you are done.
6. Press the large, red do strengthening synthesis / 強化合成する button to synthesize the base character with the materials.
There is a brown button which says auto-select / 自動選択 in large text. Clicking it lets you quickly select character synthesis materials for synthesizing. The left-hand side of the button has an icon with text under it saying currently selected / 設定中. Clicking that lets you choose which materials to use.
If you choose Manyus, higher rarity (= higher experience output) Manyus are chosen before lower ones. If you run out of higher tier Manyus, it will start using your lower tier ones. If the knight will reach max level (always assuming Great Success does not occur), lower rarity Manyus will take the place of higher ones if a lower tier one is all that is needed. For example, a knight that is two levels away from max level may have one 100 y/o Manyu and a 5 y/o Manyu automatically selected for her because using two 100 y/o Manyus would be a waste.
If you choose Ampules, the automatic selector will select as many Ampules as the character can receive. For example, a knight that has 97 Life Ampules applied to her would have only three Life Ampules automatically selected because that reaches the max of 100 Ampules for one stat.
If you choose Ampules, but the character already has 100 Ampules of the type you've chosen, the auto-select button will instead select High Ampules.
Notes and Strategies
Level ups increase your flower knight's stats, although there are some other things to be aware of. Reaching the maximum level is a pre-requirement for evolution. The maximum level of a flower knight depends on their rarity and whether or not they have been evolved or not. The maximum leveled flower knight you own will also be the maximum level of the flower knights used by comrades in arms.
The cost of synthesis equals (the flower knight's level) * 100 * (the number of materials used), so you can save money by synthesizing in large batches while the flower knight is still low leveled. In particular, a 5 or 6 star flower knight can be fed ten, 100 Year Old Manyus with matching attribute at Level 45 at base form, or at Level 60 at evolved form to reach their maximum level and save the most amount of money to reach the maximum level.
Event flower knights are not earnable from gacha, so your goal will not to be to synthesize the flower knight with herself but rather to synthesize her with her equipment and skill flowers. All event flower knights are designed to be full specced this way. By contrast, gacha flower knights of rarity 4* to 6* can be difficult to near impossible to full spec because they don't have the blessing of equipment and skill flowers.
A good number of low rarity flower knights share skills, so new players may benefit from reaching skill level 5 with these flower knights. 5* and 6* flower knights don't share any skills, so you must resort to using Kodaibanas (and Skill Flowers for event flower knights) to increase skill levels if you can't get a duplicate flower knight.
Defense Ampules increase defense, but they also increase the chance that the flower knight will defend. They also reduce more damage due to defending.
List of Flower Knights With Common Skills
The following flower knights have the same skills. Synthesize them together to reach skill level 5 without needing to worry about earning the exact same flower knight multiple times. This is not intended to be a full list of skills.