Bertram

Weapon: Steadfast Sword+ (+Def) / Carrot Cudgel (+Def) / Unbound Blade (+Def) / Slaying Edge (+Def)

Assist: Positional Assist

Special: Bonfire / Ignis

Passive A: Sturdy Stance / Atk/Def Solo / Other Atk/Def Skills 

Passive B: Quick Riposte /  Lull Atk/Def / Dull Close

Passive C: Atk Smoke / Atk/Def Menace / Rouse Atk/Def

Sacred Seal: Atk/Def Solo / Atk/Def Form / Mystic Boost / Quick Riposte

• Bertram can run a generic but nonetheless effective enemy phase build with particular specialization versus physical enemies. With the listed skills, he should be able to tank and retaliate against the majority of non-blue, non-magical enemies with great confidence.
• The number of swords Bertram can run on a tanky build is unfortunately a bit short-handed compared to lance and axe users. 
  • Fortunately, Steadfast Sword came easily summonable via Tanith in the same update and suits the build's purposes well by granting an easy Atk/Def +5 and penalty neutralization — Bertram is simply required to end his turn near an ally for these effects. You can also run its seasonal and Grail-available variant, Carrot Cudgel, if Tanith is a particularly rare pull for you, but from a resource saving perspective it's better to wait for her. 
  • Unbound Blade is a bit difficult to get as it's exclusively on a five-star flier at this time of writing, but if you get get your hands on it, it spares Bertram the effort of running Lull Atk/Def or Dull Close in his B slot as Unbound Blade basically offers the same bonus negation effect in exchange for being away from allies. 
  • Slaying Edge offers no additional stats, but its CD -1 can be rather useful at times as his Specials fire off with better frequency. In particular, Bonfire can activate on any single counterattack with outsourced Breath support (like Brave Lucina's Geirskögul).
• We focus on building up Bertram's Attack and Defense to zero out as many physical attacks and retaliate effectively as much as possible. This pretty much narrows Bertram's A-slot and Sacred Seal choices to a variety of Atk/Def-boosting choices you can afford to give him. Double Atk/Def Solo, for instance, is pretty easy to recommend as it grants a meaty amount of stats simply for ending Bertram's turn away from allies, and also pairs well with the Solo condition of Unbound Blade. Ideally, however, you should aim for Sturdy Stance 3 to get that nice Special blockage effect and reduce Bertram's chances of getting beaten by burst damage. 
• It's recommended to have Quick Riposte somewhere in the build to increase Bertram's chances at killing in retaliation with a follow-up attack; Bertram's likely too slow to make much of his own naturally. You can simply run it as a budget option in the B slot with a stat Sacred Seal, but if that's taken by Lull Atk/Def or Dull Close, running it in the Sacred Seal is also just as good. 
• Upgrading Bertram's native Rouse Atk/Def to Atk/Def Menace is an ideal plan as Menace performs a similar function but also decreases nearby enemies' stats while being less strict in positioning than Rouse. For a more reliable penalty spread option (as Menace can hit the wrong thing sometimes), good old budget Atk Smoke has got your back. 

Weapon: Ninja Katana+ / Brave Sword+

Assist: Positional Assist

Special: Luna / Galeforce / Bonfire

Passive A: Swift Sparrow 3 / Surge Sparrow / Death Blow / Atk/Spd Solo / Atk/Spd Push / Atk/Def Variations of mentioned skills for Brave Lance

Passive B: S/D Near Trace / A/D Near Trace / Lull Atk/Def / Lull Spd/Def / Hit-and-Run

Passive C: Rouse Atk/Def / Flexible

Sacred Seal: Atk/Spd Solo / Heavy Blade / Blade Session / Swift Sparrow / Death Blow / Sturdy Blow / Atk/Def Solo

• Among other current Sword cavaliers at this time of writing, Bertram's base 37 Attack, acceptable Speed, and lack of PRF weapon actually makes him rather qualified as a good Brave weapon nuke. For comparison, his Attack is only second-best to Legendary Sigurd and Seliph, and his base 27 Speed, while definitely not a hallmark by any means, reaches an acceptable usage level for Ninja Katana, allowing him to somewhat overcome his awkward follow-up issues with other forms of Atk/Spd investment. 
• Of course, you can simply run the tried-and-true Brave Sword, which offers the same forced double and blitz damage but encourages focus on pure Atk or Atk/Def skill investment due to its Speed penalty. 
• Overall, the build, skills, and playstyle are all quite simple: invest in Bertram's Atk (and other relevant accompanying stats) as much as possible, and knock the socks off of anything that Bertram can leverage his increased attack range against safely. 
• If you're running Ninja Katana, other Atk/Spd skills should take advantage of the additional Speed and possible chance at quadruple-attacking foes; Swift or Surge Sparrow are direct complementary choices with the player-phase nature of the build. Atk/Spd Solo and Atk/Spd Push notably allows Bertram to avoid being dead weight in Enemy phase, if you can't help avoiding leaving Bertram in enemy territory for some reason. Likewise, doubling up on the aforementioned skills or running Blade Session make simple and effective choices in the Sacred Seal.
• With Brave Sword, it's better to focus on Atk purely with Death Blow, or run Atk/Def modifiers such as Atk/Def Solo if emergency dual phase or a stronger Bonfire Special proc appeals to you.
• In addition to that nice extended attacking range, Bertram's cavalry status also grants him the ability to run the Near Trace or Lull skill lines as his choice for a B passive slot. Both provide a nice substantial stat debuff to two stats of choice in addition to either providing post-combat movement or dulling enemy buffs, respectively. At any rate, if you're not running Hit-and-Run as a poor man's Near Trace, what has been said above similarly applies here: 
  • S/D Near Trace or Lull Spd/Def with Ninja Katana other Speed-investing skills. 
  • A/D Near Trace or Lull Atk/Def on Brave Sword-centered builds; Lull Atk/Def in particular pairs well with Heavy Blade to put Attack comparisons in Bertram's favor and thus activate certain Specials, especially the longer-cooldown Galeforce, more quickly and reasonably. 
• Bertram's native Rouse Atk/Def fulfills the build's purposes well by granting him a simple Atk/Def bonus when he's not near other allies. It's difficult to recommend anything else outside of the insanely expensive Menace or Joint Drive skill line (and those are if you REALLY love the guy) if staying within budget is a concern.

Weapon: Armorsmasher+ (+Def)

Assist: Positional Assist

Special: Bonfire / Ignis

Passive A: Sturdy Stance / Atk/Def Bond / Other Atk/Def Skills 

Passive B: Quick Riposte

Passive C: Rouse Atk/Def

Sacred Seal: Atk/Def Solo / Sturdy Stance / Atk/Def Form / Atk/Def Bond

• At low investment, Bertram can make himself useful as a situational armor check for modes like Arena Assault. Bertram's decent Speed stat actually benefits him here, as the combination of a natural follow-up plus Quick Riposte allows him to break through some low-Speed armors with notable follow-up prevention skills, such as Wary Fighter and especially Armored Wall from the ever-menacing Fallen Edelgard. 
• Unless the matchup is particularly overwhelming, Bertram should be able to take out the majority of armored units with one Quick Riposte-powered round of Armorsmasher — a cheap and accessible necessity to beat some of the new tougher armors in the game.
• Other than the described skills, much of the build retains elements from the other enemy phase build: simply build upon Bertram's Attack and Defense in his remaining A and Sacred Seal slots, run either Bonfire or Ignis for powerful burst damage, keep Rouse Atk/Def for large and pretty self-sufficient buffs, and finish off with whichever positional assist you believe suits your allies in general.