shares more or less the same agenda on the trip,
watches out for and takes care of me while I do the same for him/her,
may not know how to read the maps well but take things in good stride even if we lose our way,
puts up with and does not mock at my idiosyncrasy,
does not complain incessantly,
gets fun out of everything – sight-seeing, shopping, eating, people-watching, exploring, anything!
is not fussy and does not expect to be waited on like a prissy princess,
shares the load – I bring the shampoo, you bring the soap,
is generous and not miserly over sharing some cost,
can walk miles and miles without having the need to constantly adhere to a fixed itinerary,
adapts to changes and makes the best out of the situation,
does not need to sleep really early,
loves the noise and also appreciates the quietude (like what XY and Minty mentioned!),
does not take photos of me from a low angle thus making my face look swollen with double chin :p
likes me enough not to get freaked out by my "just out of bed look".
Add on to my list of the PTC. Keep the voxpops coming :)

My PTCs! We miss Taipei :(