Separate your contentloading from the class that holds your object data.

public static class Art
{
    private static Dictionary<string, Texture2D> _textures;
    
    public static Init(ContentManager cm)
    {
        //add each texture to the dictionary of textures with the path.
    }
    
    public Texture2D Sprite(string path)
    {
        return _textures(path); // ofcourse do some checking if a path exists in the dictionary etc.
    }
}

Now your paths are no longer causing duplicate content to load because you decoupled that so:

// Actual behavior object the game interacts with. May be serialized in a savegame file.
public class Character
{
    public Character(CharacterData data)
    {
        Level = 1,
        Hp = 234,
        Name = data.Name;
        Texturepath = data.TexturePath;
    }
}

Next to solve the rivals- you shouldn't declare "new" characters in each character you create. Create a pool of characters first. And then appoint rivals.

Weapons can be a mixed bag. Let's say you have a weapon like "excalibur"- one exists, but any character could own it- so you may want to create it outside of the character. On the other hand, each weapon could be unique in the sense that it has its own durability stat- in that way you can create one when needed (when a character is created, a chest is opened etc).

Separate your contentloading from the class that holds your object data.

public static class Art
{
    private static Dictionary<string, Texture2D> _textures;
    private ContentManager _cm;
    
    public static Init(ContentManager cm)
    {
        _cm = cm;

        //add each texture to the dictionary of textures with the path- you were sure to load anyway.
    }

    public static void LoadAsset(string path)
    {
        if(_textures.KeyExists(path))
          return;

        _textures.Add(path,_cm.Load<Texture2D>(path)); // add your checks for the existence of file etc.
    }
    
    public Texture2D Sprite(string path)
    {
        return _textures(path); // ofcourse do some checking if a path exists in the dictionary etc.
    }
}

Now your paths are no longer causing duplicate content to load because you decoupled that so:

// Actual behavior object the game interacts with. May be serialized in a savegame file.
public class Character
{
    public Character(CharacterData data)
    {
        Level = 1,
        Hp = 234,
        Name = data.Name;
        Texturepath = data.TexturePath;
        Art.LoadAsset(Texturepath);
    }
}

Now in your draw function you can point to the Art.Sprite() to pick up the texture;

Next to solve the rivals circular references- you shouldn't declare "new" characters in each character you create. Create a pool of characters first. And then appoint rivals afterwards.

Weapons can be a mixed bag. Let's say you have a weapon like "excalibur"- one exists, but any character could own it- so you may want to create it outside of the character. On the other hand, each weapon could be unique in the sense that it has its own durability stat- in that way you can create one when needed (when a character is created, a chest is opened etc).

Separate your contentloading from the class that holds your object data.

public static class Art
{
    private static Dictionary<string, Texture2D> _textures;
    
    public static Init(ContentManager cm)
    {
        //add each texture to the dictionary of textures with the path.
    }
    
    public Texture2D Sprite(string path)
    {
        return _textures(path); // ofcourse do some checking if a path exists in the dictionary etc.
    }
}

Now your paths are no longer causing duplicate content to load because you decoupled that so:

// Actual behavior object the game interacts with. May be serialized in a savegame file.
public class Character
{
    public Character(CharacterData data)
    {
        Level = 1,
        Hp = 234,
        Name = data.Name;
        Texturepath = data.TexturePath;
    }
}

Next to solve the rivals- you shouldn't declare "new" characters- if these aren't new characters (or weapons). Let's say you have a weapon like "excalibur"- one exists, but any character could own it. Create a pool of characters and weapons first. And then appoint weapons and rivals.

Separate your contentloading from the class that holds your object data.

public static class Art
{
    private static Dictionary<string, Texture2D> _textures;
    
    public static Init(ContentManager cm)
    {
        //add each texture to the dictionary of textures with the path.
    }
    
    public Texture2D Sprite(string path)
    {
        return _textures(path); // ofcourse do some checking if a path exists in the dictionary etc.
    }
}

Now your paths are no longer causing duplicate content to load because you decoupled that so:

// Actual behavior object the game interacts with. May be serialized in a savegame file.
public class Character
{
    public Character(CharacterData data)
    {
        Level = 1,
        Hp = 234,
        Name = data.Name;
        Texturepath = data.TexturePath;
    }
}

Next to solve the rivals- you shouldn't declare "new" characters in each character you create. Create a pool of characters first. And then appoint rivals.

Weapons can be a mixed bag. Let's say you have a weapon like "excalibur"- one exists, but any character could own it- so you may want to create it outside of the character. On the other hand, each weapon could be unique in the sense that it has its own durability stat- in that way you can create one when needed (when a character is created, a chest is opened etc).