@Milk Without conversion roll (which is unsure if it will be included in EP6 exchange), melon is an average pet (for bossing) with high pp consumption and bad for mobbing.
If you're just starting with summoner, I suggest you to focus on maxing at least one 13* Jinga ( you can obtain it from collection folder) with Sunny/Very Sunny personality and Light element.
If I recall correctly, you get one 13* wanda with very sunny personality from story quest but I don't remember the element, so make sure you have at least one wanda with light element, then change the rarity from NPC with the 13* and use its personality, then evolve it into Jinga being careful to mess the personality in the process.
Another suggestion I can give you, is to have at least one of each pet so you can do Pietro's weeklies for free sweets.
If you have Photon Spheres in excess, you can buy 12* EXP eggs from the pet NPC to level them up faster.
When you have common eggs in excess, you can use them as fodder to increase stats of a pet faster (beware: you basically lose all the XP):
- unhatch an egg
- synthesize 5 eggs (with stats) on it
- return the fodder pet to egg via NPC
- synthesize it on the main pet
(you can optimize it further to work around the synthesis cooldown)
Don't forget to level up summoner class in all your alts until you get the 10th gift from Pietro, which is a Support Roll (that's the only way to get them, unless you want to pay absurd prices to people that exploit alts to obtain unlimited amount of them).
A fast way to do this process on alts is to run "Suppress Za'udan" from ARKS Quests with Pietro as companion (it takes 50 runs +/-) as soon as you obtain his partner card (you need to complete his "A Parent's Pride" CO).