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Leo has two different stances: Normal, and Brynhildr. While Brynhildr has useful combo extension/combo starter/ground combo tools, Brynhildr's juggle ending options are very lackluster.
Grounded Combo Theory:
The main goal of any Leo combo is to get a knockdown or Siegesparade restand. The secondary goal is to do as much damage as possible while not jeopardizing or reducing the quality of the knockdown It's not always possible to end in a meterless knockdown if Leo is too far away, but compared to most Guilty Gear characters Leo excels at obtaining maximum reward off every hit.
The "juicy" normal to get to is c.S because that does a good amount of damage through his 5H.
Leo has hugely rewarding meterless combos against crouching characters. Crouching opponents experience one more frame of hitstun than standing opponents. Leo gains two great combo routes against crouching opponents: 6K > 5P, and f.S (or 2S) > 5H. The difference in damage is dramatic.
Leo's most rewarding combo tool is 6H because of its ground bounce and high damage value. In any optimal juggle situation, you would never pass up a 6H.
Most of Leo's combos can end in an air juggle into the restand j.236H Siegesparade move. Some Leo players prefer to take the Siegesparade restand, but others dislike it depending on matchup because of the inability to block afterwards. An alternative is to end combos with H Eisen which gives a hard knockdown but doesn't allow for any setups besides general meaty attacks. H Eisen combos also do the most damage, but the difference is generally less than 5 points.
In some combo routes H Eisen knockdown will be possible but an air combo into Siegesparade will fail because the combo was too long. While Xrd uses actual combo time and not number of hits to deteriorate combo hitstun, generally j.K/j.S/j.H > j.236H only works at around 10 to 11 hits on the combo counter. When Siegesparade is not possible, some Leo players will end with j.Hj.D (which will not combo) to allow the opponent to tech high in the air and tech chase. That guessing game has its own problems, especially since the opponent can choose when, which way, and whether to air tech at all.
Except in very short combos (5 hits or less) or with RISC bar, Leo's j.K will always lead to more damage than j.S with 1 or 2 hits. j.Kj.S(1) will lead to higher damage than either j.K or j.S alone. Every Leo air combo should usually end in j.H because j.H has the highest damage but no combo options besides ending in j.236H.
Side note about ground bounces in Xrd: Ground bounces will bounce the opponent higher when the opponent starts higher. A 6H that hits an opponent high up will bounce the opponent higher than one that hits an opponent right before they land on the ground. A lot of combo confusion stems from different 6H timings being used on different characters. For example, against light characters, the goal should be to hit 6H fairly late so that they don't bounce high above you. However, for Potemkin, if you delay the 6H then the followup combo will need to start with 2P and may no longer lead to a good knockdown. On a "high" ground bounce, it is possible to negate part of it by hitting the opponent right after they bounce.
While you can adjust Leo's combos to be easier and less character specific, you give up damage to do so. In my opinion, Leo's potential only really shows when his hits lead to the monstrous damage of which he is capable.