As soon as Osu/Mesu hits 99 units in weight, it'll morph into Debutchi, the "chubby" character, and take up the entire screen (as you probably have already seen). All you need to do is play games with it until it hits 89 or so.
I personally have not played with my HK Commemorative, other than to take it out, place batteries in to see if it worked, and check for the certificate of authenticity in the box. Normally, I buy to play, but I want to save this one in the box for now, which is why I've bought a separate P1 out-of-box to play with. Anyway, there are many different ways to carry your Tamagotchis around. Depends on how creative you want to get. Have you taken a look at THIS thread?
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