I am confused by the difficulties that people apparently encounter here.
It starts with the original question about a triode (whatever that is).
In Eeschema (KiCad's schematic editor), just press "a" for add and then start typing "triode" in the search box.
The link that jpanhalt posted about making a symbol in KiCad should suffice in that area. I do recommend to start with a copy of an existing triode and then modifying it to your own insights.
Starting with an example shows you already what it looks like, and it keeps the form of the symbol in line with the other library symbols.
Creating and managing schematic symbols (ditto for footprints) is quite easy in KiCad. If people perceive it as "more difficult than in eagle" then it very likely is because they have years of experience in that other program and have forgotten about the learning curve in that program, and have to learn it again in KiCad because it works a bit "differently".
I switched to Linux some 6 to 8 years ago and for this I needed a Schematic + PCB program that works with Linux. I tried Eagle back then (and about 6 others) but I found it an annoying program to work with. I wanted to add a 0.2 Ohm resistor to a schematic, and Eagle would not let me do that.
Then I looked further and found KiCad. The way that KiCad works fits better with the way my brain works. Back then it had quite a bit of rough edges and some things (such as library management) did not work properly, but KiCad has made leaps of progress since then.