I think TekuTeku is more like that Dancing Moon Bunny VP Sony made in the early 2000's. The one you connected to your Walkman/mp3 hardware/other music source and the more you listened music with the VP attached to the audio cable and the bunny dancing in the screen, the more well the bunny stayed in shape.
If you didn't connect your Moon Bunny with audio device daily and listen music the bunny started to gain size and became chubby.
TekuTeku Angel suppose to be similar, with the difference that instead of connecting the deviice to audio source/portable player you walk around with the TekuTeku VP attached to your belt or in your pocket and you can set goal for steps like example 10'000 a day. And if you reach that goal daily the "Angel" will stay in shape and grow to cute character. If you don't reach the goal you set or leave your TekuTeku "unwalkeded" for many days it becomes chubby and grows to more ugly looking creature.
(This is just my assume, I'm not really sure does it work this way)
There comes also software for Nintendo DS that includes some sort of exercise calendar that lets you keep record of your daily amounts of steps and maybe help your calculate how many calories/joules you have spent with walking?