Units gain EXP after completing quests. Since they can be played in full auto, you can auto farm for some exp. Doing this will also increase their bond, so you can get some extra rewards.
After progressing through the story, the "Training" section of quests will open up. Daily Training Quest is where you can farm for Experience Tomes, Lucres, and Echoes.
There is a limit to how many times you can do these quests, as shown in the upper right. It's recommended to make your units stronger by doing story quests first, which will level up your characters and get materials to upgrade equipment and memory gems before trying high difficulty training quests.
View an ally's information and select the second tab to level up a unit with Experience Tomes.
Experience Tome (S) gives 10 exp.
Experience Tome (M) gives 50 exp.
Experience Tome (L) gives 100 exp.
It's often better to get EXP for Allies by doing quests in the game since it's more efficient to use the tomes on Knitwits so they can be ascended.
Before departing on quests, you can use boost items to increase Lucre or EXP rewards that you get from battle.
You can use both boost items at a time, but only 1 of each boost can be used per run.
Best Quest for Your Buck Memory Gem gives an increase to exp gained from battle. This bonus applies to all Allies that are in the party, so it's a handy Memory Gem to use for any Ally that's in the sub-slot.
Increasing the Memory Gem's level won't increase the bonus gained. It will only increase when you Unleash it by using extra copies of the same Memory Gem.
How to Unleash Your Units
Using duplicates of an ally, you can unleash the unit in order to unlock and/or upgrade abilities. Doing this also increases their max level. Each character can only be unleashed up to +6.
On this screen, if you don't select any units, you can tap the + icons on the left to view all the available Unleash upgrades the unit can get.
Benevodon Spirit Icons works as a substitute to Unleash any character that's the same rarity. A 4★ Benevodon Spirit Icon is given for free to all players when starting the game, which allows you to Unleash a 4★ of your choice once.
Specific Benevodon Spirit Icon also exists to Unleash whichever ally that it shows. The Icon for -Strong and Sweet- Primm is available as a reward from the bingo mission.
What is Ascend?
Ascending lets you increase the rarity of the Ally, which will also increase their max level cap. You can ascend them from the character page and go to the 4th tab and select "Ascend".
In order to Ascend an Ally, they need to be at max level first to be eligible. You will then need another Ally that's the same rarity as the one that you want to ascend. The amount of Allies needed to Ascend also depends on the rarity of the base unit.
Knitwit Icons are essentially substitute materials made just for Ascending Allies. Since they also have a rarity, you can only use them to Ascend Allies of the same rarity. They are obtained as rewards from missions, bingos, or story mode.
How to Ascend High Rarity Allies
Ascending high rarity Allies can be quite costly, but there are 2 other ways to do this other than to get another 4★ Ally from the gacha.
One of them is to Ascend low rarity Allies, but this can be quite a slow and tedious process. It takes a total of 12 2★ Ally in order to make 1 4★, so you'll need 48 2★ Allies to Ascend a 4★.
You can use the sub-slot in your party when auto farming for quests in order to get exp to Ascend them. You can also equip Best Quest for Your Buck to get some EXP increase during battle.
Since there aren't any ways to obtain a free Ally, the only way to get them is mostly through summons or events.
Knitwits can't be brought into battle as an Ally, but you can level them up using Experience Tomes in order to Ascend them. This is useful since they need less EXP to reach max level compared to a regular Ally, making it easier/faster to reach 4★.
Awakening is the finishing touch upgrade to finalize an Ally, making them from a 5★ Ally to 6★. There are 4 requirements before an Ally can be awakened:
Must be 5★
Fully unlocked Mana Board.
100 Awakening Orbs are also needed. It's a limited item that's obtainable from rewards.
Once Awakening is done, the Ally's rarity will become 6★ and level 100 as well as some stat increases.
What is Mana Board?
Each character has their own mana board, and upgrading them gives the character more stats. The materials required to unlock them depends on the element of the character.
You can unlock the first row of each stat on the mana board at level 10, then there is a 10 level gap to unlock the next row until level 70, where the gap becomes every 4 levels until 99. Unlocking more of these will increase an Ally's power level.
If you use "Unlock All", the game will open up the Mana Boards evenly, until they all reach the same level before unlocking the next row.
This isn't recommended if you only want to focus on a few stats. A purely physical character won't really need INT, so you can manually level up important stats first instead.
Materials to upgrade Mana Boards are obtainable from Altar Trials. There are altars for each element and they are available every day.
Luna & Dryad Materials
For Allies that are Moon or Wood elements like Honeycomb or Mousseline, you can only get their Mana Board materials from the shop exchange.
From the game's menu, select "Shop", then Dryad or Luna Coin Trader.
To exchange for Dryad materials, you need Lumina (Light) Mana Board materials.
To exchange for Luna materials, you need Shade (Dark) Mana Board materials.
It's a 1:1 conversion for both Moon and Wood exchange, so they aren't more expensive than raising any other Allies.