Both Krunner and Kickoff in Kubuntu return a list of found items after the user has entered at least three characters in the search field.
Usage on Synapse shows that returning a list after one character onwards is faster and efficient.
I would like Kickoff and Krunner to behave similarly, by making the return a list of items after entering the first character in their search field.
How can I achieve that?
No, this settings can't be changed in default setup. Krunner and Kickoff are designed to display search suggestion after at least 3 letters in input. So, you can't change them because there is not configuration option to do so.
See this Wikipedia Article about Krunner.
Also see this documentation in userbase.kde.org which says, krunner requires at least 3 letters to be typed to match for a partial search.
A bug report was filed against this behavior in https://bugs.kde.org, though it's status is still (7-9-2012) unconfirmed, the first response was
So, it seems KDE developers are not willing to change this behavior. I think the only option is to change it yourself and then re-compile.
Note that, both Krunner and Kickoff was designed in this way
Unlike Synapse, which tries to match items based on any input, Kickoff and Krunner are designed to match after you've inserted enough information to match with.
My suggestion would be to report a bug against the two as a wishlist item, since there are currently no options (that I know of), that allow you to change this.