To get Yohan as a consistent contact in your phone you need to walk into the storage locker area of your nightclub. After you do that, you'll be able to call him whenever you want and he'll help you source special cargo for the nightclub (mission has a 20 minute cooldown time).
Characters like Victor Vance, Johnny Klebitz, and CJ experience tragic endings that reflect the dark nature of the game. The worst ending in GTA 5 is killing Michael, which leads to Franklin being declared dead by Trevor and Michael's family, leaving him constantly watchful for danger.