Bond mysteries/passives only unlock when two conditions are met.

1) You have the ninjas it requires attached to them

2) The total assist level of the attached ninjas is 25 or higher

oh, I though that it requires 4 as it shows :/

So if it requires 25 lvl which means a lot of sources I've to put in... it comes to question if that is ever worth it ?
There was many changes since I've been playing last time and I heard somewhere that orochi has been nerfed :x

anyway, thank you for quick reply :)

If you're planning to get a bonds skill but you only have few resources for spending then i will suggest you to invest for Minato Jonin's bond even if you don't have him .. At least someday or one day you get him, you already have his Bond Skill, since you c*so use the upgraded cards to one of your used ninjas to give additional power/atts to you .. Nowadays, you will not and cannot see anyone using Orochimaru (except for the akatsuki version which only few using him), so having limited resources (f2p) and spending for his bonds, is not a great idea ..

It's not worth. At least until you make pvp-viable team with orochimaru (which doesnt and wont exist to my knowledge) and have other 3 ninja slots (12 cards in total) maxed out. As well as bond mysteries of other ninjas you could potentially use.

Required assist link level refers to required ammount of ninja cards (ukon, kidomaru, tayuya, jirobo- thus lvl 4). If you check minato jonin's description it requires 2 (kakashi kid and obito kid), and so on.