07/31/18 - 4:00PM ~ 08/13/18 - 3:59PM JST (Quests)
07/31/18 - 4:00PM ~ 08/20/18 - 11:59PM JST (Shop Exchange)
With Judith being obtainable in the Daily Login Campaign for the Tales of Vesperia Anniversary event, Rita and Raven are obtained through the event's quest challenges. Simply grind medals and hit the target count to obtain Rita and the needed materials to awaken her into a 6 star unit. Raven's awakening material at 30,000 medals is the final Rita/Raven related material grind, via target score of medals.
Raven, however, can be purchased in the event shop with the medals you obtain. Four of him are available so that you can unlock his max level without having to use any of your leveling materials. Also in the shop, are the final material upgrades (located under Raven) needed to awaken Rita and Raven into 6 star units.
You will need a total of 27,000 medals to be able to purchase all Rita and Raven related event items for full unlocks of their potential strength. Make sure to grind out the quests these coming days, so that you can awaken them. There is a high chance that this event will not return next year, as it celebrates 10 years of Tales of Vesperia!