I wouldn't rush out immediately and get another cat - I would leave it a few weeks at least if you can. You may find your other cat will come out of his/her shell more now the other cat isn't around - especially of the cat you lost was more assertive.

Plenty of TLC, as already suggested, lots of fuss and attention and interaction by way of games to distract him/her.

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Kaz

For kittens, joy is for the asking and smiles are for the giving