According to Mystery Tour there are only 6 onion quests that lead to the Promise Badge. 1.Truth, Justice, and the Onion Way! - Justice Badge
2.Know One's Onions - Scroll - Blaze Spikes
3.Inspector's Gadget! - Heko Obi
4.Onion Rings - Bouncer Club
5.Crying Over Onions - Star necklace
6.The Promise - Promise Badge
The stats on a Promise Badge are as follows according to this webpage Type:Pendant Slot: Neck Def: 3 HP: 10 MND: 5 Jobs: ALL Races: ALL Level to Use: 48
From the research I have done the last of these quests (the one resulting in the Promise Badge) is the only one not already summarized here. You must complete the other ones first then talk to Kohlo-Lakolo (G - 5) in Windy port again, then go talk to Chamama in Windurst Waters (F-11). She asks for Shoal moss (according to Myster tour) or shoal weed. Head to Kuftal tunnel and kill recluse spiders for the Shoal weed and return it to Chamama,she gives you the invisible man seal. Take the seal to Kohlo-Lakolo for the Promise Badge.