meet the knight guys at the very beginning of game? on the left of the gacha machine in Adventurer's Path: Gathering Point. Then you must have 5000 yen and buy it. Lastly, just brought it to home (without being badly injured).
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well, have it, then in random days, you will see your sis will be playing the JSV. Just, click on her, then choose games options, (also must have favor and desire lv up about 3-4 hearts? I dunno, just try xD). you will have option to get her on your lap, tease her while her playing, and that's it....