Today, I guess nobody would use "Hallo mein Lieber" to initiate a serious flirt with somebody (using "du" in this sentence sounds wrong to my ears). This sentence has (if said to an adult) an ironic undertone, in which case you could say that someone initiated flirtatious joking about it if this makes any sense.

Same goes for "Hallo mein Süßer / meine Süße".

Today, I guess nobody would use "Hallo mein Lieber" to initiate a serious flirtatious with somebody (using "du" in this sentence sounds wrong to my ears). This sentence has (if said to an adult) an ironic undertone, in which case you could say that someone initiated flirtatious while joking about it if this makes any sense.

Same goes for "Hallo mein Süßer / meine Süße".

Today, I guess nobody would use "Hallo mein Lieber" to initiate a serious flirt with somebody (using "du" in this sentence sounds wrong in my ears). This sentence has (if said to an adult) an ironic undertone, in which case you could say that someone initiated a flirt by joking about flirting if this makes any sense...

Same goes for "Hallo mein Süßer / meine Süße".

Today, I guess nobody would use "Hallo mein Lieber" to initiate a serious flirt with somebody (using "du" in this sentence sounds wrong to my ears). This sentence has (if said to an adult) an ironic undertone, in which case you could say that someone initiated flirtatious joking about it if this makes any sense.

Same goes for "Hallo mein Süßer / meine Süße".