Ser Jon Darklyn is a knight of the Seven Kingdoms. And a Knight of the Kingsguard. Perhaps Jon Darklyn may be the best sword fighter in all of the Seven Kingdoms, Jon Darklyn 'The Dark Sword'. He has exceptional swordsmanship-power, defeating Ser Arthur Swyft, The Sword of the Evening. He is alike to his other Kingsguard brothers, especially Edmund Moore but the two of them diverge into different paths as result of the story. Jon Darklyn is captured for most of the war, while Edmund remains by Aeyron's side.
Jon is capable, very capable. Aeyron chooses Jon to take command of 7K men to go to GoldGift and he warns Ser Lewyn not to become arrogant like Ser Jamie Casterly did at his age. Jamie eventually went on to kill his own king. One of the pivotal representations of Jon's character at the start is when he meets with Jamie Casterly and we see the dogma in Jon's voice and the lack of concern from Jamie's. 'Are you missing this tower? Shame, only Kingsguard can enter”', he allows himself to be fully devoted to the Kingsguard, it's meaning and it's oaths - which Jamie completely crossed. Jamie doesn't care for his comment while Jon is affected by Jamie's because it questions Jon's position in the Kingsguard - '[Jon] had killed Ser Arthur but Jon always wondered what would’ve happened if they faced off fresh'.
Jon's position as Kingsguard is slowly beginning to be questioned in each chapter of his - '“Your Grace with all due respect I am of the Kingsguard, sworn to protect you to stay by your side I must". Jon doesn't think it's in his duties to lead an army for the King but the King is sure that he must do it, in Jon's mind the Kingsguard should just be by the King, protecting the King - controlling an army goes against his idea of being a Kingsguard; but to also be a Kingsguard is to also obey your king and so he does. He leads the army although he doesn't want to.
What do you think qualifies you to be a Kingsguard? Selflessness? Bravery? Courage? Jon exhibits these qualities while he leads his army. He takes 5 thousand men with him, knowing that they're all going to die, but he has them with him so that they may allow the others to escape - 'Men! Many of you will lose your lives today, many will be wounded or injured! You not win glory here, your sacrifice will be all but forgotten but your duty is to hold the line! It is your duty as well as mine!” He assessed his troops all shuddering. “This is the duty you owe the Kingdoms and your duty begins now!”' He doesn't fight for glory, he fights without selfishness.
From being next to Aeyron for more than 5 years to nearly spending the whole war without him. Jon is captured by Nestor and thrown in a cell. You could imagine it's enough for a person to have a crisis, how does the king do now? Who is taking care of the King without him? If he is doing fine, what is the point of Jon being there then? He's horribly tortured under Nestor's watch and is terribly alone for the first time in his life. 'Jon had already decided to give up, no one would come for him, Aeyron had probably figured he was dead now and he was soon to be.' The knight loses all hope, and he accepts that Aeyron had maybe moved on from him. So in his mind, his position as Kingsguard is already far from jeopardised, it may be taken away. He grows nostaglic for the Kingsguard position, thinking back to how he got it in the first place. Through killing Ser Arthur Swyft. He thinks that maybe he isn't worth the position of Kingsguard for failing the King and being captured - 'The rain shows what I am now, nothing but a flash in the pan. One rain drop that is overshadowed by all the rest.'
Thankfully Jon is rescued by Soniya Cross, and almost similarly to Ignis, she brings some sort of colour to Jon's life. But it is like meeting the colour purple after only knowing yellow for your whole life, she's something new and beautiful and she's enticing, and she saved your life let's not forget. Soniya can also fight and she's very capable at it - 'Everyone knew I was a Kingsguard and off limits so no one tried however Soniya was the first one to actually try to turn me.'. She also speaks plainly to Jon, something he may have missed after all those years in the capital, 'Jon did not know whether she was joking or being serious, he abruptly stopped staring at Soniya with fascination, a few seconds later she realised he had stopped'. Soniya is fun, she asks Jon questions that he could have never answered before such as - '“You cannot marry but you can have fun on the side?”' Her presence in Jon's life is a stark contrast to the previous months he had, stuck in a cell and she admits it to him - 'And so do you Ser Dark, you cannot live in black and white while the world around you is vibrant in colour. There are fruits to try, people to meet, places to see.”'
Along the line Jon and Soniya develop a keen-bond, Soniya saves his life for a second time against the Red Baron but in the first moments there is still some air of awakwardness between them two and it's fascinating to see. Does the Kingsguard knight resolve this awakardness and stick to his code, or does he let them become closer? ' She cares too much and I have shown her I care too little. If only we can talk but she refuses, thank the Gods I’m of the Kingsguard because I do not understand women.' Jon is indecisive when it comes to Soniya but she is not, she orders only one room for them in an inn knowing what it means.
There comes symbolism with Jon's return to King's Landing. He ofcourse lost his Kingsguard armour to Nestor, so wears the Red Baron's armour with a Kingsguard cloak on. It represents his failure to commit to either one, the Kingsguard or Soniya. I don't think Jon is ready in book one to choose between them two. He does however make a choice in book one, during the Battle of the Reach he leaves the Kingsguard and fights alongside Soniya.
Before the Battle of the Reach Jon does assume some responsibility as a Kingsguard, discussing with Lewyn how to improve Lewyn's fighting technique. And he originally fights as a Kingsguard, protecting the king at the start of the Battle of the Reach. Jon does feel guilt when calls are called that claim 'the king is dead'. 'Jon was left ashamed, he were part of the Kingsguard sworn to protect the king but instead he had given into his selfish ways and gone to fight alongside Soniya'.
Like I said earlier, Jon Darklyn is not ready to make a choice between Soniya and The Kingsguard as of Book One.