The term ruler refers to any character who is in charge of a realm, no matter the size.
This is a high rank in any hierarchy, though not necessarily the highest if your realm is part of another realm. Gameplay at this level is often dominated by diplomacy and management of your vassals.
Keeping the realm together and, if possible, expanding it is the primary responsibility of a ruler. Whether they rule through consent or oppression, making sure that the settlements that make up the realm remain with it ultimately determines its fate.
Internally, the ruler manages the realm positions (see Other roles) and this in turn determines the government system and feel of a realm. It also determines many of the responsibilities of the ruler. In some realms, they might just be the official head of state, while in others they may be a tyrant with absolute control of everything. Monarchies, republics and other forms of government lie somewhere inbetween.
On the outside, the ruler is also responsible for the realms Diplomacy.
Plenty. There will always be someone who is complaining. The members of your realm if you are not expanding, your neighbours if you are. Your friends and allies when you do not support their wars, their enemies when you do. You will be labelled a weakling if you hold back and a powermonger when you don't.
Playing a ruler is the ultimate challenge in this game, and can be both thrilling and exhausting. If you as the player find it drains you of energy, that is normal and means you need to do one of two things: Either you need to delegate more and do less yourself or you need to step back to a less demanding role for a while. There are many similarities between being a ruler in Might & Fealty and being a leader in real life - it is rewarding, and it takes a lot of energy.
As long as your realm is but a sub-realm, even as ruler you still have other rulers above you. So for the ambitious, the final goal will always be to be the ruler of a sovereign realm, with others below, but nobody else above him or her.
There are the usual ways of achieving this - like inheritance or conquest - but you can also move not yourself, but your realm up the hierarchy, by taking on more vassals and sub-realms and growing in size and power until you can either declare independence from your superior realm, or even reverse the tables and exchange positions. This may or may not require a bit of civil war.
One player wrote an excellent summary of how to attract and keep loyal vassals. Read it here on the forum