This make a good trick question for the students.
I ask them this every year. Almost all of them repeat what they got taught in anatomy the year before, that it is an inverter of the foot. However, every year there is usually one or two smart students who have worked this one out. The correct answer is that it could be both.
The tendon of the tibialis anterior muscle inserts into the medial cuneiform, which is, on average medial to the subtalar joint axis, so has a lever arm to be an inverter of the foot.
BUT, thats relative to the average position of the subtalar joint axis. In a lot of people the axis is more medially located, so the insertion of the tendon of tibialis anterior is going to be lateral to the joint axis in these people. It then has a lever arm to evert the foot.
Think about this...this is just one tendon relative to one joint axis. What about all the other tendons relative to the variations in all the other joint axes. The joint axes taught in anatomy are the mean position of the axis. There is always going to be standard deviations about those axes mean positions that will change tendon lever arms and change foot function ... and you thought we knew how the foot functioned!