Line of Sight is not the same as Cover and whether or not there is Line of Sight depends on the specifics of what the object is.
Squares on a grid are not a measure of the size of a creature, they are instead a measure of the area a creature can control:
A creature's space is the area in feet that it effectively controls in combat, not an expression of its physical dimensions. A typical Medium creature isn't 5 feet wide, for example, but it does control a space that wide. [...]
A creature's space also reflects the area it needs to fight effectively. [...]
(PHB > Combat > Movement and Position > Creature Size > Space)
So even though the base of the mini might take up the full 5ft square, the creature on the base doesn't. As a result, it is possible for a creature to have half cover, depending on positioning, but no Line of Sight to the creature.
To see how this interacts with Line of Sight and Cover, we need to go to the DMG.
The rules for Line of Sight are (emphasis mine):
[...] pick a corner of one space and trace an imaginary line from that corner to any part of another space. If at least one such line doesn’t pass through or touch an object or effect that blocks vision — such as a stone wall, a thick curtain, or a dense cloud of fog — then there is line of sight.
(DMG > Combat > Using Miniatures > Line of Sight)
Cover on the other hand (for a grid of squares) states (emphasis mine):
[...] choose a corner of the attacker’s space or the point of origin of an area of effect. Then trace imaginary lines from that corner to every corner of any one square the target occupies. If one or two of those lines are blocked by an obstacle [...], the target has half cover. If three or four of those lines are blocked but the attack can still reach the target [...], the target has three-quarters cover.
(DMG > Combat > Using Miniatures > Cover)
So for Line of Sight, the lines to the creature need to not touch an object, while for Cover the lines need to not be blocked by the object. Whether or not it "touches" the object will of course depend on the nature of the specific object.
So, if in the first example the pillar is a 5ft square block of stone, then your determination is incorrect. There is no Line of Sight to the creature and they have half cover.
If instead it is a 1 ft wide pillar that the PC is behind, then there is Line of Sight to the creature and it has half cover.
Wraithful Smite inflicts the frightened condition on a creature:
[...] if the target is a creature, it must make a Wisdom saving throw or be frightened of you until the spell ends. As an action, the creature can make a Wisdom check against your spell save DC to steel its resolve and end this spell.
and the frightened condition states that the creature:
[...] has disadvantage on ability checks and attack rolls while the source of its fear is within line of sight.
Wraithful Smite ends after the first of:
- 1 minute
- The enemy's concentration is broken
- The affected creature uses it's action to end the condition
So, regardless of Line of Sight or not, the frightened condition remains on the creature. But the disadvantage effect on the creature is not in effect while they can block Line of Sight.
Importantly, however, the second bullet point of the frightened condition is still in effect (regardless of Line of Sight):
The creature can't willingly move closer to the source of it's fear.