This is a popular pairing throughout the fandom. One of the first moments of this pairing is the dance they had in episode 6 at Celeste's birthday party and the pairing has grown in popularity due to fan speculation and other moments, such as when Lilette actually says "I love you" to Alfred. They first met in the ruins of the Old Captial outside of Perstity in episode 2, where Alfred and Arthur originally awoke after being sent to Rukassia.
This is another popular pairing in the fandom. It is hinted by fans that Lilette and Arthur have feelings for each other. A moment in this pairing is when Arthur was hitting on Lilette in the special episode known as drinking games. He tends to get rather upset when Francis hits on her because of what fans specualte to be jealousy. They originally met in the ruins of the destroyed capital. Arthur was the first to discover that Lilette was a demon.
This pairing is one that is known mainly for Francis' constant flirting, even though at times it is only to annoy Arthur. Moments of this pairing are generally Francis being Francis, a well known one being in episode 19 when Francis offered his room to two of the Axis Powers and he would bunk with Lilette. They first met when he was found flirting with random women by Arthur and Alfred in the town of Aska at the start of episode 4.
This pairing is semi-popular in the fandom. It's first and very well known momenyt is when Rondo meets and kidnaps Lilette in episode 12 while the Allies are in Molgalva. There in an empty shop he kisses her against her will. Later, in the tower that guards the Lightning Alter in episode 18 1-3, he meets up with her again and starts licking her face, which was probably due to a dog-like curse from Melody.
One of few of the popular pairings from the original series of Hetalia, it gained popularity in the Tales of Hetalia universe around the start of the series, especially when Alfred was attempting to ask Lilette to dance in episode 6.
Another rare original Hetalia pairing, it is mostly onesided with Francis constantly making hints and practically flirting with Arthur, like in the original Hetalia series.
This pairing seems to start at some point during the time that the Axis Powers were in Nifhliem, and hints of it grow stronger after Flat leaves the demon race and joins the Judges. Their first meeting is after Feliciano and the rest of the Axis agree to help the demons in Nifhliem.
After Melody discovers that Rondo appears to have a crush on Lilette, she is pointed out to seem rather envious in the situation, mainly by Flat. After Rondo assassinated te infant princess Preludia years ago, he was put into Melody's custody and she has since been controlling and torturing him with curses for his punishment.