The first time you play the game, you will have to go through the tutorial.
For the cutscenes section of the tutorial, you can skip it by tapping the "Menu" -> "Skip" in the upper right part of the screen.
After going through the battle tutorials, there will be 1 free summon that you can continuously keep drawing until you get the unit that you want. In this free first summon, you can only get 3 star versions of a character, so there's no need to keep retrying for a 4 star.
Once you've reached the main home screen, tap the present box on the right side of the screen and claim all the presents. This gives 1 guaranteed 4 star character ticket, and 5400 Spirit Crystals, which is enough to do 2 10-summons.
If you aren't satisfied with what you got, you can go to the main menu by tapping the button on the upper right, then select "Title Screen".
In the game's title screen, tap "User Support" and then "Delete User Data". Deleting the user's data this way is faster than re-installing the whole game since you won't have to download the APK and additional game data when you want to reroll.
After you've deleted the game data, you can start a new game and there will be a prompt asking if you want to skip the tutorial right after you pick and name your main character.
This puts you right into the very first free summon of the game where you can continuously summon until you get the 3 star character that you want.
Once you are satisfied with your summons, consider saving your account by setting up a data transfer information.
Disclaimer: This is a very early and rough estimation of the tier list.
Release Celebration Harvest banner has increased rate to get a 4 star, and a rate up for both Riesz and Duran. Keldric's banner has everything the Release Celebration banner has but with the addition of 4★ Keldric and Aim for Wind Acorns Memory Gem.
If you're planning on rerolling until you get a good start, then the rate up of the characters won't matter much, so it's recommended to go for Keldric's banner instead so you have a chance of getting Aim for Wind Acorns.
Each banner also gives a different Harvest Trader Points that can be exchanged for a 4★ character. You'll need to do 20 10-summons to exchange for 1 character. Harvest Celebration banner has Duran and Riesz as the available options, while Guided by the Great Tree has Keldric and Aim for Wind Acorns.
Thrilling Blade Flash Duran has the highest HP, STR, and CON at max level. His skill deals damage, reduces enemy CON, and is a hard form of crowd control. His other skill increases his already high CON to help him survive. Altogether, this makes him potentially a future-proof unit.
Aim for Wind Acorns is an ATK boosting Memory Gem for a Wind element Ally, and it's the best in slot Memory Gem for Riesz or Keldric. Even if you don't have them, there are 2 3★ Wind event Allies given for free that can reach +6 and will benefit from this.
Magic Power in Bloom Angela is a mage with high INT and Water damage for a skill, but short range ST and normal attack, as well as low HP to be quite squishy. She's a great character to bring out for some burst damage by using up her MP, but struggles in auto-play or long drawn-out fights.
Headed for the Home Village Popoi is a hybrid that deals phys damage on ST, but magic damage on skill. Exploder magic attack can hit pretty hard, and the other skill increases INT, but his attacks don't recover MP due to weapon type. His decent INT and STR potentially makes him a good unit to use Magic Spirit Magic for some high burst.
Overcoming Myriad Farewells Sumo is the only 4★ Ally out of the 6 available that can heal. He also deals decent physical damage and can be tanky, making him a great unit to handle high level contents or for coop as support.
Encounter Sprung from a Seed Shiloh high ST damage and is slightly tanky because of high CON + a buff that gives more CON and Super Armor. Has a unique effect at +3 unleash that gives a small heal on self and paired ally for some self-sustain.
Guided by the Great Tree Keldric is a Wind element bruiser. With a focus on multi-hit attacks, he works similarly to Riesz. The main difference is that Keldric's other skill gives him CON for more survivability. His ST is also a 10-hit attack, which will immediately give a damage bonus and it's great against multiple enemies.
Pride of Amazon Riesz is the most damage-focused Ally of the available 4-stars. She has an ATK increasing skill and a multi-hit attack to help her quickly build ST so she can deal burst damage. This will also help increase the combo counter to increase overall damage, which can be maintained by her dodge that deals damage too.
Most of the 4★ Memory Gems are also in the B category since they're all mostly good, but not as important or game-changing as getting Allies, especially since some of the 3★ and 2★ Memory Gems have similar effects to the 4★, just less values.